I can totally relate to what you're saying.  

Age definitely has something to do with it.  But it doesn't necessarily mean our taste has become simpler or more casual as we've gotten older.  

It's just that we've got more experienced with game in general.

Like BCrayfish said, "I've been shooting aliens for twenty years, slapping on a new coat of paint doesn't really do it for me anymore."

There are literally hundreds of game formulas and templates in our head, which we naturally collected over the years.  It's rare to find something new or interesting these days.

Most games are very predictable in essence.  With only a few hours of play, I can already sense, with reasonable accuracy, where the game is going and ends up.  



Seeking something new, we, experienced gamers, first look for bigger and more complex titles.  This is understandable.  We hope that big games will surprise us.  

But after a while, say 30 epic games later, it becomes painfully obvious that the bigger-complex approach doesn't necessarily offer us something new.  

After this, we usually become less willing to invest a lot of time learning or mastering big complex games because it's usually decreasing returns - the more time and effort we put into the game, the less rate of return (Fun / Effort) it becomes.

Few games have a good rate of return.  

A game like Boom Blox is very efficient to have fun because that it has a flat learning curve and yet is entertaining even for the been-there-done-that crowd.  

Mastering the game mechanics in fact takes a lot of time and therefore the gameplay has a depth, but studying it doesn't really feel like a job because of its simple design.  



So I play Boom Blox.  

Then a younger gamer comes, looks at me, and says

"Ha! You ARE a casual gamer!"  

and I can't help but feel the irony.  

Hawkeye said:

 When does this start happening? I am 17 now, and most games feel exciting and fresh, and least for a while. Brawls freshness ran out after 2 months, which is a lot worse than Melee (felt novel for YEARS) but its still there..