Ages and age categories are given for only a very small subset of creatures; the core player races, a handful of other player races, and the occasional monster. Ages are not given for any animals and it is not reasonable to assume that their age progression is intended to model real life progression, though it is clear that they do age and and have the normal age categories all creatures have (it's much more clear in the actual text in the book than in this SRD link, though, but basically, everybody has 'Adult, Middle Aged, Old, and Venerable' as age categories as a general rule, and animals don't have any indication that they are exceptions to this). It is thus entirely unsurprising that how becoming an animal companion affects a creature's natural lifespan is not addressed anywhere in the rules. Technically this means it doesn't extend the lifespan, RAW, but basically animal aging is outside the scope of the RAW and your GM will have to come up with a subsystem (or at least generate/'research' a lot of content) if they want to include it in their game.
Your companion living longer naturally won't really break anything mechanically, and there are a bunch of cool ways to handle it, so it'll probably be ok. Just be aware that 'animal companions live longer than normal' will be weird and probably shouldn't be a thing (since animal companion status is temporary and you can swap animal companions whenever, and are supposed to do so when changing environments. The rule would thus mean that animal companions don't actually live any longer, since they normally stop being companions first and also that if they had exceeded their natural lifespan and you stopped having them be your companion they'd fall over dead from old age), whereas 'serving as an animal companion extends an animal's natural lifespan by one year for each year served, at the cost of one year of the master's lifespan' or some such would be fine, since the added age remains when the animal is no longer a companion.