I played French Horn throughout my time in high school at Lisgar Collegiate Institute under the direction of Robert Hursti. University did not permit much involvement; life since also. I finally got around to bringing my horn to Texas , so I could start working my lips back into shape...
Many years away did not result in being unable to play again; the really interesting thing I have discovered is that there are some breathing exercises particularly involving playing very long notes at fairly high pitches that resulted in being very quickly being able to play remarkably high material.
My embouchure still suffers somewhat, but so long as I'm willing to inflict a bit of aural pain on anyone that happens to be around, it's not too tough to get the high end back. Indeed, I've played some material that was higher than anything I ever attempted when in high school when I thought I was in my best shape, lip-wise...