I disagree about the youngers. My kid had just started club ball when they made the age switch and it proved to be a tremendous disadvantage. He went from being the tallest and fastest to at best middle of the pack. Lots of coaches in the early flights needing to get promotions also play run and shoot ball so that extra leg speed is of benefit for both the attacking players and the defensive players.I don't think the statement about smaller boys at a young age is true. Lots of great flight 1 teams with smaller players at young teams. Smaller boys play different positions and have to be more agile. The 7-9th grade years is when size becomes an issue because puberty comes later for some and may create a 1.5 feet height difference. If your player has not developed great footskills and quickness, the puberty years can be tough. If your player is having a hard time hanging with bigger players during puberty, it's not a bad idea to drop a flight or two until they can handle it. Some small players can hang and some can't. Be realistic and observant as a parent. It's ok to drop off to another flight to protect your child and prepare him for the future. Don't let flight dictate the puberty years, let growth and development do it. When your child is 16, that's when scouts start paying attention unless they're an early bloomer and already 6 feet by 14. Some kids will get noticed first but others will remain noticed once they hit their stride. Scouts know about puberty too.
If your player was never fast or quick...it's very unlikely they will develop that skill after puberty. They will get faster and quicker but so will the other players, therefore your player will still be slower than others. Yes, there are exceptions to this but it's a very small %. Uncoordinated small kids generally grow up to be uncoordinated tall adults.
My son wasn't always the tallest or biggest but he was quick. When he grew legs, he became fast because he was always able to move his legs quickly. I've seen most of the boys do the same through puberty.
To see the difference, his private elementary school ran an annual jog athon where the boys and girls competed separately but all the age classes competed together. The winner for the last 10 years (with 1 exception) has been a fifth grader. Yes there are some 4th graders who finish higher than the fifth grader, but ON AVERAGE the older the boy the better they perform. He finished second the jog a thon his final year, first was a birthday roughly his age, beating out another 5th grader who is very small and agile but is 8 months younger and just simply couldn't keep up with either of them in the multiple laps even though he dominated the first lap, trained cross country, and trained for the event (while the two winning boys, basketball and soccer GK respectively, didn't)