3 sentences

1. 看这匹马的口齿，正当壮年。

   kàn zhè pǐ mǎ de kǒu​chǐ, zhèng​dàng zhuàng​nián.

   Looking at this horse's teeth, it is in its prime.

   Source: Dong Chinese Dictionary

2. 他正值壮年，精力非常旺盛。

   tā zhèng​zhí zhuàng​nián, jīng​lì fēi​cháng wàng​shèng.

   He is in his prime and has great energy.

   Source: Dong Chinese Dictionary

3. 他正值壮年，却全然不懂男女风情。

   tā zhèng​zhí zhuàng​nián, què quán​rán bù dǒng nán​nǚ fēng​qíng.

   He is in his prime but knows nothing of romance.

   Source: Dong Chinese Dictionary