Board Thread:Questions and Answers/@comment-35718513-20191020022158/@comment-10330822-20191026063607

He didn't. Oetsu stated that when Yhwach invaded Soul Society in the far past and first fought against Yamamoto, he looked as "Old Man" Zangetsu looks now. The "teen" Yhwach is what Tensa Zangetsu looks like.

The "appearance" of Yhwach is consistent, the real question is why Yhwach still ages if his longevity is enhanced by his powers. Because it looks like he just "ages slower".

As for the other Sternritter, my guess is that the Sternritter are given special abilities and have their ages slowed because of their pieces of Yhwach in them. The other Wandenreich are never stated to have any sort of special aging powers. In fact, my original guess is that they just reproduce normally and that is how they've existed in secrecy all these years.