I'm going to go out on a limb here and say 'no'.
It's rather like Purification of the Festering Wounds - your body is strengthened while the spell lasts, when the spell runs out we have status quo.
This would also be consistent with the phrasing of the CrHe 15 guideline.
No ritual. Why should it? The fact that the change is drastic in its consequences does not make a spell to be ritual necessarily. In the mechanics it is assumed that you are "perfect" just in the moment that you reach maturity. Before you are just growing up, and after that you are decaying. So "perfection" (unaffected by creo or Perdo spells) lasts but a moment. Weeks at most.
Hmmm.... a plot idea using a magus hit with a longevity ritual JUST at that moment (parens obsessed with age; one of the CrCo specialists of the OoH) just starts boiling.....
I would say it does not require a ritual. First, the guideline does not say it requires one, while it does says it requires one for (for instance) Creo Corpus healing magic. Second, there are a number of guidelines, progressively more difficult, depending on the time it takes to achieve maturity; if it were ritual magic there would be only one guideline, for momentary. The fact that it actually takes time to reach maturity shows that the magic accelerates a natural process instead of replacing it. At the end of the spell, the process has still occurred, so the plant stay mature. Finally, Covenants p.79 presents a spell that use another variant of the same guideline, sprouting a seed in one day or night, and it is not a ritual.