I'm running a game that blends VtM V20 and 2ed. In that game I have a few considerations:
For player characters I think that it should mostly be dictated by roleplaying experience more than age. If they want new rituals, it becomes a matter of finding the resources and then enough game time passing. We usually treat this sort of thing as an extended roll. For my group, this exact scenario hasn't come up because we don't have any serious magic users.
For NPCs I have two tracks. For NPCs in the city's "milieu" (which means they exist for the players to interact with but aren't major agents in the plot without player intervention) I mostly build the character according to the standard character creation rules and then give one ritual per dot in any magical path (not discipline) of equal or lesser level than the character's overall corresponding Discipline rating.
For major NPCs (those that drive the plot with or without player intervention) it's pure storyteller need. The rituals themselves are really not balanced well within or between sorcerous Disciplines, so I don't like limiting such important characters while creating them-- the builds end up too fragile to deal with players taking the story off the rails, and useless rituals are a waste. I do try to stick to the one-ritual-per-magic-path-dot, as with milieu characters, but I don't assign all of the specific rituals right away.
I'll give major characters rituals that the character "would know", in the sense that if the character regularly does something a ritual supports or makes possible, then the character knows it and this "uses up" one of the allotted rituals. The rest are fluid until they are needed. This flexibility remains until the chapter in which the players first meet the character ends (more or less), and then the rest of the fluid rituals are assigned to ones I think fit their concepts and backstories or they are lost. For high-level rituals I might require the NPC to devote resources to discovering/learning them "off camera", but that ties in with some other house systems I use.
TL;DR: The rituals an NPC knows (and their number) should support the needs of the story first and foremost regardless of any guideline. After that I aim roughly for one ritual per path dot, loosely adjusted for the NPC's access to ritual knowledge, inclination to study, and relevant Attributes, Abilities (primarily Intelligence and Occult, but others sometimes apply), and Disciplines.