expose your kids early... if they like it, then look at doing it further.
Yes, it has gotten younger and younger and it is not necessarily a good thing. I firmly believe the adults have totally screwed this up!
that said, my oldest is all things baseball. when he was 8 (he started at 5), he had two kids on the team (still a developmental team not an elite team) who had never played - they quit after that one season because they were sooooooo far behind the other kids in skill level that they were completely frustrated. I held the line and we didn't do anything organized until kindergarten, which made us oddballs. we have continued to hold the line and insist on different activities in different seasons (no fall baseball, no winter hitting league); that is not his choice but it is ours as parents because we don't think that much of one sport is good for him. however, my son just paid the price for our attitude in that he was cut from an elite team. not because he wasn't good enough but because we weren't dedicated enough. ****sigh***** we've found a good team that's not quite so competitive so hopefully it will all work out. we filtered the information so he's excited about the change.
my daughter is in soccer. we just quit a team because they had moved to year-round playing. again, we just don't think this is healthy development. we've found a fantastic coach who also thinks two seasons of soccer is plenty at 9 years old but we had to search. I think many parents are afraid to rock that boat - look what happened to my son.
be aware that by waiting, your kids will NOT be able to play most high school sports. their skill levels will simply be too far behind. they may be able to play jay vee, they may be able to do things that don't require "development" such as cross-country, but they will have an extremely slim chance of playing at the varsity level for sports like soccer, baseball, football, basketball. even golf and tennis - most kids start by 8. also be aware that for some sports, the college scouts don't even look at the high school teams - in my state (Colorado), the scouts pretty much only look at club players for soccer.
they should be experimenting with sports by early/mid elementary school to see if there's anything they really like. plus there's so many other lessons that sports teaches that don't happen in school. you just have to navigate the crazy parents and the general craziness that sports has become but it can be well worth it. try the Y or the local rec district rather than club if you can, as they tend to be more low key.
realize that your kids will be absolutely fine without the early sports but there are the trade offs. If you're okay with that, then listen to the crazy parents without a word, laugh inside, and move on. if you really want your kids to have more sports experience in junior high and high school then see if you can find some low-key 6-8 week seasons at the Y. know that sports will not be much of an option at 11-14 other than things like swim, golf, tennis because it moves to the school versus club for many things and if they don't have the skills, they won't play.
Most kids will not play high school much less college or beyond despite what the crazy parents think - they'll be burned out! our goal has always been to do just enough to keep their passion piqued and fed, develop their skills just enough that when they hit high school, they can play at an elite level if they choose. My kids have the talent. we are doing our best to nurture the skills to go with the talent while keeping it sane and giving them a childhood that includes a lot of play and free time. It can be done, it just takes a strong will and a thick skin (really! - my son is cut because of us! - yes, this is a sore point right now)
good luck!