It kinda depends on what kind of strength. For things where the athlete is rather lightweight/small and does some kind of fast/twitchy power moves you peak out

It kinda depends on what kind of strength. For things where the athlete is rather lightweight/small and does some kind of fast/twitchy power moves you peak out somewhere before 30. But when all you need is pure strenght you have 30 to 40 year old world champions in many sports (you still have to start training when a lot younger to make it though) for example in power lifting where you have a lot of todays world champions born in the late 70s/early 80s. One of the advantages (and disadvantages) of sending 18-20 year olds into battlefield is that if they die society spent less money/effort into teaching them some other valuable skill that would have been lost without getting the benefit out of it (due to the person being dead)