That's a good question! I can honestly answer in three ways.
1. I personally think his age in the game is somewhere around 13 y/o. So, if his story "stops" when the game ends, then I guess that's how old he'd be now. Story, and therefore the world and his character, can't progress once the game is finished, right?
But that's kinda boring, so it's time for answers #2 and #3.
2. TNC came out nine years ago—in 2009—so if Mike was 13 when the game was released, then he'd be 22 now. I like this idea the most since the games already kinda have a heavy connection to real life/our world anyway, so this is just kinda a logical continuation of that theme. (At least in my opinion.) In a way, the games tell his story, but he's still separate from them: It's kinda like his relation to the Raposa world in the games!
also something that kinda (but not really) supports this would be jowee's 4th-wall-breaking quote in the first game about all the raposa + their world being within a DS system. More of that real-life connection stuff?
3. Not my favorite, but it's also possible Mike could be pretty old right now. Well, that is if you believe any artwork made by the games' artists is "canon".