I think kids should play everything. Compete at least some (see the certain Istitute rant site for a good read on why real competition is good for kids) but the mix is great.
I played the classic american 3 before dropping them in high school, but loved the best days, pee wee football, playing hard, hitting, running all night... and ended up still competing further, though, with a college ultimate team that went to collegiate nationals, then boxing, winning a novice golden gloves, and now weightlifting. I played for some great coaches, and some bad ones. I'm sure you did, too.
But I'm not sure of your post - are you having a "If only I had played tennis younger, I'd a been a contender or champion!" moment? Well, get over that if you are, and love the sport now, and be glad you're fresh to it.
In the end pro sports will reject 99.9% of the people in the world for lacking the talent, drive and luck to make it, so realistically, I think we should approach sports for their other benefits - learning, fitness, social development, teamwork, character building, social interaction, and a love of the physical.
So you're about 30 now? Love it man. Tennis is fun! Play as hard as you want. Be glad you're able to play now, and didn't get burned out on it like you did with baseball.
"Morning, Putski eats it, noon, Putski eats it, night, Putski eats it. Putski loves!"