The question shouldn't be whether you are too old for airsoft, but whether you are too young to be taking up a lame-arse traditional old-man's sport like Golf.
You are only as old as you think you are.. and there are far worse ways of going about a mid-life crisis, than airsoft.
45 here as well. Started in 2011. Don't worry about your knees. I started doing this sport as a Redneck form of physio-rehab in the midst of two hip replacements. This year, i might even get to shoot my guns, more than i was hugging trees for support in the past two years.
While the median age (as i have observed) is fairly young for airsoft players in Canada.. there are older players. With airsoft gaining in popularity, you can reasonably expect that more older players will be finding their way to fields.
It's a matter of hooking up with the right group of players, and the right field .. a lot of the commercial fields do cater to the youngin's.. but there are other fields that are a little less publicized in terms of attracting that crowd - more semi-private, if you will.