Spirit has no canon age, and nothing is known about how phils age, how long their lifespans normally are, or how long the days/years on their planet are. So it's not something I worry about.
I do see him as being older than I am though, since he's portrayed as an adult in canon and I'm turning 19 in two weeks. But viewing him as the equivalent of a 25 year-old human isn't any easier or harder for me than viewing him as the equivalent of someone in their 40s-50s, so it'll probably be a long time before I feel the need to age him up, if at all. When I insert myself into his universe, I tend to be in my early-mid 20s, though that's for experience and independence reasons rather than age ones. I don't really mind any age gap between us since we're already different species, so it's not the same as a relationship like that between two humans.
There are species in his canon that can live for hundreds (and, in come cases, thousands) of years so it's possible that he could age very slowly, however I'm not sure how I feel about that, it's a bittersweet idea.