I can go and find credile scholarly articles that will argue the exact opposite. and I did just that.
"He concludes that age-mixed play offers opportunities for learning and development not present in play among those close in age, permitting younger children to learn more from older playmates than they could from playing with only their peers. In age-mixed play, the more sophisticated behavior of older children offers role models for younger children, who also typically receive more emotional support from older kids than from those near their own age. Age-mixed play also permits older children to learn by teaching and to practice nurturance and leadership; and they are often inspired by the imagination and creativity of their younger playmates." from Psychology Today
"The problem is that age segregation works well for students in the middle of the group in terms of ability. However, those who are developing faster or who are older must be pulled back to the pace of the whole group and are often frustrated by their inability to move forward; those who are developing more slowly or are younger may feel left behind and lost." Headsup.org via
Charles D. Bernstein (Ph.D)
"5 UNEXPECTED BENEFITS OF MIXED AGE MONTESSORI CLASSROOMS; More effective teaching, More effective learning time, Greater interest in learning, Stronger foundation and unlimited learning potential, and Leadership skills." By Ms. Marty Shepard
"Children today may have few opportunities to be around children younger or older than they are, especially in a fairly informal environment. Managed well, an OSHC service gives children many of the same opportunities that an extended family might provide. Children can learn so much from older children, and older children can gain confidence and satisfaction from the opportunity to ‘teach’ younger children and demonstrate, often to an admiring audience, their skills and talents. In an environment that is supportive, older children can increase their selfawareness through opportunities that come from being around younger children. They may gain some insights into how they used to be when they were younger. They can also come to appreciate the strengths and wisdom of younger children."
I really wish I still had access to my University academical journals because I know the list can just go on, but this is what I was able to get from Google in five minutes. This is NO one size fits all answer for our athletes not by Doctors, you, me , or USASF. There will be exceptions and out-liers to ever possible scenario.