My kids took different paths to the same result. Both played rec ball in their preteen years. My daughter (softball) had zero inclination to practice on her own or play for a travel team when the rec season ended. At nine and ten my son played on a community sponsored travel team after the rec season. It was essentially the CR all star team. At eleven and twelve I operated a AA level USSSA team in a Sunday doubleheader league. The roster was fifteen potential LL all stars from our league. My son hit and hit off his Little Tikes tee from the time he was eighteen months old. He always wanted to practice on his own (with me).
Starting with 7th grade my daughter played in a travel program known for their 18u players going on to college softball. The objective was to advance her skills to become a high school player. The thought of playing college ball never crossed anyone's mind until she grew (shot up) in 8th grade and made varsity freshman year. The travel program moved her up to 18u freshman year. She verballed after soph year. That's somewhat late for D1 girls.
i created a USSSA Majors team for my son at 13u. The Jr Legion coach's philosophy was 13yos don't play (at all). They sit the bench and pay their dues. I felt my son was better than half his players at thirteen. No way was he going to sit for a year. My team was thirteen of the best former LL all stars from our district.
The objective was to turn the kids into high school players. Along the way for three years I chatted with the coaches of academy programs as often as possible to build a bridge for the players to get to showcase ball. Eventually the team was obtained by a new baseball academy. The purchase price was the kids played for free one year. Several kids including my son switched programs for showcase ball. Some of the kids stayed with the program.
i felt playing top competition starting with 13u might have got my son to varsity a year early. I don't believe it changed the big picture outcome. Playing 16u after freshman year got him ready for high school ball. 16u got him exposure for several showcase team invites.
To net it out I would say the most important thing in middle school is best coaching and playing time. Once a kid can handle the 60/90 field he should play up as far as he can handle. By 7th grade he should at least be playing grade appropriate and not down a year in an age group. My son has a May birthday. The deadline change occurred allowing him another year of 12yo LL. He passed and moved on to 13u with his grade and friends.
Note: Due to the success of my son's LL all star teams based on the extra competitive travel experience and starting a more elite level 13u team each year following my son through LL started the same process. It destroyed the Legion program. None of the better players play Legion anymore. At the Legion meeting is a voodoo doll of me they stick with pins. Ten years later they're still ripping me behind my back. What they don't realize is they ruined the Legion program with their approach. You don't sit players who can play. The Jr Legion head coach was in love with himself. He always bragged about his 20+ years of coaching experience. From what I saw he had one year of experience twenty times over. He knew it all. He never grew with the game.