It's a real test of strength so you're at your peak in your 20s, and after that it's a slow decline.
When your physical endurance starts to drop, if you don't have any skill or leadership qualities you just become a burden.
どういう文脈の使い方でしょうか。プロサッカーなどですか。
「体力勝負だから全盛期は20代であとはズルズル後退。」＝＞
"It's a real test of strength/stamina so you're at your peak/prime in your 20s, and after that it's a slow decline."
体力 = strength; stamina; physical endurance
体力勝負 = a test of strength
全盛期 = peak; prime; golden age
ズルズル後退 = a slow retreat; a gradual decline
「体力もなくなりスキルも統率力もなければお荷物に。」＝＞
"When your physical endurance starts to drop, if you don't have any skill or leadership qualities you just become a burden."
統率力 = leadership; leadership qualities/skills
荷物 = baggage; a burden
It is a matter of how physically strong you are, so it's the best while we're in our 20's but after that we just go down.
It is a matter of how physically strong you are どれだけ体力があるかということが重要（問題）なのであって
so it's the best while we're in our 20's 20代のうちはよいけど
but after that we just go down. そのあとは後退するだけです
アメリカ合衆国 
日本 
