at the peak

at the peak of — на вершине (успеха/славы);

Example 1: Аnn is at the peak of her fitness.

Example 2: We saw a victory by an athlete at the very peak of his skating career. (CALD)

see also
[в зените ]
[в расцвете]
fit as a fiddle
alive and kicking
full of urine and vinegar
full of beans = full of energy, active, cheerful

past one's prime (He is now past his prime.)
[over the hill]