It's really the same question with any hobby or fandom. The simple answer is that age has nothing to do with it - it's something that brings enjoyment. No one gets asked when they're too old to follow their favorite sports team, for instance, and these are also games kids play (and they have even crazier conventions, or so I hearā¦

). Oddly, much of that fan base technically may be less connected as many fans don't even play the sports recreationally. The social climate may be a bit different in the US than in Australia, but here we have a fairly healthy nerd and geek subculture that's becoming fairly prevalent. Digimon just tends to be a bit more obscure even in this culture, which is fine. It's sort of like how everyone seems to like Superman or Batman but often don't even know important heroes such as Doctor Fate. Also not Digimon, but there are a few senior citizens in my area who play PokƩmon Go (and I know some professors who do as well, for what that's worth).
But there also well may be a psychological component to your friends' reaction. As humans, we often draw attention to what we see as the flaws of others so that our own flaws get passed over. If your friends care so much about what you do in your spare time, it more likely suggests they have their own secret fandom or other obsession or even general insecurities. Definitely don't let their thoughts, or apparent thoughts, keep you from something you like, As long as it brings some form of enjoyment, you're not too old.