His peak period was in his thirties.
This civilization reached its zenith.
絶頂期 stresses the very topmost, dramatic point of success, while 全盛期 covers the whole stretch of time someone remains at their best.
peak of a financial bubble; bubble peak
Compound of 絶頂 (ぜっちょう, 'peak, summit') and 期 (き, 'period, time').