He looks markedly different in the first episode of Season 6.

Here’s a photo of Troup in 1967 on Dragnet, age 49:
https://qph.cf2.quoracdn.net/main-qimg-791bf394737c2be3722320ae5b8722d6

Here he is five years later in 1972 on Emergency!

https://qph.cf2.quoracdn.net/main-qimg-e05bb985273115ae1a0d422554a3f3ea

And 1977, the final year of the series:
https://qph.cf2.quoracdn.net/main-qimg-fc5a3e621e9ed637102527c16842b533

I don’t think he looks different, just different poses in the photos.

He looks slightly older as he ages, same as most of us.