Just wanted to make sure you knew it wasn't mandatory to take the same age as the other. It is only coincidence that they all picked the same starting age. As it's a trade-off between a bit more initial xp vs having to get a longevity ritual sooner, it doesn't unbalance things at all. And Ars Magica isn't about being balanced anyway! Older magi can be vastly more powerful... or not, depending on how you evaluate that power.
I'm suggesting a flaw from HoH:MC only because it is essentially the Hermetic story equivalent of the mundane Heir flaw. Magi do need cash, sort-of-ish, but that is usually generated by the covenant rather than from personal property. It provides very little advantage to have a mundane forge in Strassburg, considering that you'll be joining the covenant of Fengheld (which is over 300 miles away), to establish a new chapter house who-knows-where (I know where it will be, but I'm not telling yet). And by the time you inherit, that source of income would probably mean very little to your magus.
So unless your character tried to maintain contact with his mundane family during apprenticeship, it is unlikely that they'd even know where you are now. Magi don't mix much with normal folks, unless they have a Gentle Gift. Which means that all you would get from the Heir flaw is very distant trouble, which would be easy for you to avoid (or hard for me to pull you into). When you are a magus, being the Heir to a mundane isn't of much interest, unless it is for someone someone very rich or very powerful politically (like a duke or a bishop).
But if you still want to take Heir, I can live with it.
Oh, stuff in the lab can still use detailed notes (lab texts, see pp.101-102), but that doesn't mean you need to know a spell. Enchanted items simply use different lab texts than formulaic spells do. The only thing you gain by knowing the spell is the Similar Spells bonus (see p.101).
My point is that "Metalsmith" is not a valid choice for a Craft ability in Ars Magica. It is too broad because it covers too many different materials (iron, copper, silver, gold, bronze, etc.)
From p.64: "In general, Craft Abilities are distinguished by the material they work with, although you may also take a Craft Ability that allows the character to work with several materials in one specific way."
So valid choices would be Blacksmith (working with iron), Weaponsmith (making weapons from any material), Wheelwright (making carts with both wood and iron), Silversmith, Goldsmith, Bellmaker (making bells of bronze, brass or iron).
Actually, just to be clear, spells are not purchsed from the general pool of xp. It's a seperate pool of 120 levels of spells + 30 levels because of the Skilled Parens virtue.