I can't think of a single band that would qualify.
Every band you listen to is 50 years old. Surely there has to be ONE.
Well...it depends on whether you're actually answering the question asked or changing it to a different question -- i.e., stuff that's "really good," as opposed to "career best."
For me:
Rush - Something in the AFTK through MP era. Nothing they've released in the "very late" part of their career (let's say VT, S&A and CW) comes even remotely close.
Dream Theater - My top 3 are I&W (1992), SFAM (1999) and SDOIT (2002). If we call "very late" the Mangini-era albums, the best of those (IMO) is no better than their seventh best album.
Fates Warning - Darkness in a Different Light (2013) is really good, but I still rank it lower than A Pleasant Shade of Gray (1997), Perfect Symmetry (1989) and Parallels (1991).
Priest, Maiden and Queensryche released their best albums in the 1980s. Yes and Genesis released their best albums in the 1970s/early 1980s.
I could keep going, but the common theme is that all of those bands' "career best" albums were released in the first half of their careers.