Since several people seem to discount the plausibility of the aforementioned grog, let me give you an example.
I'll first calculate the grog's xps with the detailed character creation, and then run the grog through a year-by-year very detailed creation that should be both realistic and yield the same totals.
Let's call her Buttercup. Her (final) Virtues and Flaws are: Custos, Affinity with Single Weapon, Puissant Single Weapon, Ambitious (become best swordsman in the world), Transvestite, Small frame.
At age 20, she has Speak Own Language 5, 45xp for her first 5 years, and then 15xp each of the following 15 years, for a total of 225xp. Because becoming the greatest swordmaster ever is her burning ambition, our house rule allows her to push her Single Weapon score up to 5+2+2+2=11 (including the Puissant Bonus). That requires "spending" 150xp, which her Affinity multiplies by 1.5 to 225xp (i.e. a score of 9), on top of which she gets the +2 Puissant bonus. So, in the end, she'll have:
Speak Language 5
225xp (including the affinity bonus) in Single Weapon
45xp + 75xp =120 xp in other Abilities.
Note that since she's custos, she can take both martial Abilities and Speak Latin.
Let's run through her story in greater detail and see if we can come up with those numbers in a realistic way.
Buttercup is covenant-raised -- either a foundling or the daughter of some covenfolk (some say she is the the daughter of a passing knight). In her covenant, like in most of Europe, warrior-roles are held by men while women are cooks, weavers etc. But Buttercup, since she was a child, has always been fascinated by weapons (in particular swords) and fighting, and has always dreamt of being Custos, fighting giants, demons and dragons side-by-side with the mages.
Let's look at her experience. A reasonable assumption would be that such a grog gets 2 seasons of exposure and 2 seasons of practice in that grog's free time, for a total of 12xp per year. That's a bit less than the "standard" 15xp per year, so let's assume that the Covenant gives its grogs and covenfolk a 7 years apprenticeship (starting on their 7th year), during which one season of practice is substituted by a season of training by someone with the relevant skill at level 6. Let's say those 7 seasons are 4 in the character's primary occupation (Single Weapon for Custos), and 1 each in 3 secondary skills of importance: for a Custos, I'd say Awareness, Chirurgy (for emergencies), and Speak Latin (to communicate with the magi). This would yield a total (for those 15 years) of (2+2+4+4)*8 + (2+2+4+9)*7 = 215 xp.
Still 10xp short. We'll deal with those, assuming that a few of the trainers are slightly better; or that the custos can usually coax another season of training from some senior covenfolk; or that the custos took part in a story (substituting story experience to exposure for one season). Of course, Affinity in play tends to yield slightly more than a 50% bonus because of the rounding, so that will play a part too. Let's look at the years in greater detail.
1-5Buttercup's first 5 years yield a Speak Language 5, and 45 xp in a number of basic skills -- let's say:
5xp in Area Lore [local covenant]
5xp in Athletics
5xp in Awareness
10xp in Brawl (she is a real bully to other children!)
5xp in Charm
5xp in Guile
5xp in Stealth
5xp in Survival
6. On her sixth year Buttercup starts "working", being bossed around by the covenfolk's women who try and figure out where her talents lie. The covenant's cook is seriously thinking about making her an apprentice. Buttercup gets 2 seasons of exposure, for 4 xp. Let's say:
1xp Cooking
1xp Stealth (avoiding chores...)
1xp Survival (she gets sent to pick herbs around the covenant)
1xp Charm (another way to avoid chores)
However, Buttercup does not want to become a cook. She wants to become Custos! An older grog has given her a toy wooden sword, with which she practices on her own, obsessively, every single spare minute (well, she also practices beating up other children, including ones several years older). She gets 2 seasons of practice in Single Weapon, for a total of 8xp, which her Affinity increases to 12xp. Including her Puissant bonus, she now has an impressive Single Weapon score of 1+2.
7. Buttercup sneaks out of the covenant, following a 10 year old apprentice who snuck out for some reason of no interest to us. The apprentice gets attacked by a wolf (or some similar minor threat) and Buttercup ferociously drives the threat away with her wooden sword. She is Custos after all! Word spreads at the covenant and, seeing Buttercup's dedication and considerable natural talent, the Turb's captain agrees she'll be treated as a boy and apprenticed as a grog (this accounts for her Transvestite Flaw). This year Buttercup gets:
2xp of exposure: 1xp in Cooking, 1xp in Awareness
5xps for the Story: 3xp in Single Weapon (which Affinity raises to 5xp), 1xp in Awareness, 1xp in Stealth
9xps for Training in Single Weapon (which Affinity raises to 14xp)
4xps for practice in Single Weapon (which Affinity raises to 6xp)
Buttercup now has 37xp in Single Weapon and 53xp in other abilities not including Speak Own Language.
8-10. Three fairly uneventful years, in which Buttercup continues to single-mindedly study the sword. Buttercup in each gets:
4xp of exposure: 2xp in Awareness from guard duty, 1xp in Area Lore [covenant and surroundings], 1xp in Speak Latin from a smattering of phrases heard here and there.
9xp of training in Single Weapon (which her affinity raises to 14xp)
4xp of practice in Single Weapon (which her affinity raises to 6xp)
At the end of this period Buttercup has a fearsome 97xp in Single Weapon (for an effective score of 5+2 including the Puissant bonus) and 65xp in other abilities not including Speak Own Language.
11-12. Two more relatively uneventful years.
Buttercup in each gets:
4xp of exposure: 2xp in Awareness from guard duty, 1xp in Area Lore [covenant and surroundings], 1xp in Speak Latin from a smattering of phrases heard here and there.
9xp of training -- one year Chirurgy (by the Covenant's physician) and one year in Awareness (by the Covenant's chief of Scouts).
4xp of practice in Single Weapon (which her affinity raises to 6xp). Yes, Buttercup still spends every free moment practicing the sword. It's her obsession!
At the end of this period Buttercup has 109xp in Single Weapon (for an effective score of 6+2 including the Puissant bonus) and 91xp in other Abilities not including Speak Own Language. The covenant's weaponmaster acknowledges that Buttercup is now a master his equal, and that she has very little to learn from him.
13 The old weaponmaster dies; with his dying breath he recommends Buttercup as his successor, causing quite a stir among the turb. But Buttercup defeats every challenger in combat, and she becomes the new weaponmaster -- the youngest ever in the covenant's history. Buttercup this year gets:
4xp of exposure: 1xp in Single Weapon (raised to 2xp by her Affinity), 1xp in Awareness, 1xp in Leadership, 1xp in Chirurgy
11xp in Speak Latin -- the Covenant's Scribe teaches her and another would-be custos.
4xp of practice in Single Weapon (which her Affinity raises to 6xp).
At the end of this period Buttercup has 117xp in Single Weapon and 105xp in other Abilities not including Speak Own Language.
14-20 Buttercup is now the new weaponmaster of the covenant. These are seven relatively uneventful years -- until the last season, when Buttercup gets to accompany an old magus and the apprentice she saved many years ago to the Tribunal. The apprentice is sworn to the code, and now a magus, declares Buttercup will be his Custos!
In each of her 14 free seasons Buttercup keeps obsessively practicing her sword, for a total of 56xp (which her affinity raises to 84xp).
7 of her "work" seasons are spent training other grogs, gaining 14xp of exposure in Single Weapon (which her affinity raises to 21xp)
6 more seasons are spent on guard duty, each season yields 2xp in non-martial Abilities.
1 more "work" season spent accompanying the magi to Tribunal. This yields 5 story xps. Since Buttercup gets involved in some friendly duels with other grogs at the tribunal, she spends 2xp in her Single Weapon Ability (which her Affinity raises to 3) and other 3xp in non-martial abilities.
At the age of 20, Buttercup has then 225xp in Single Weapon, and 120xp in other abilities. Quod erat demonstrandum.