I'm in my 50s and just started riding fixed late last year when I built up my ghetto Schwinn conversion. Love it. I take it on all but the fastest club rides. It's mostly flat around here -- however:
On two club rides over the same hilly stretch of road I used my fixed one day & my geared roadie the other. I swear going up the hills was easier on my fixie! On my road bike I was gasping like a beached fish, while on the fixed I was tired but not overly distressed. (OTOH, spinning at 30+mph on the downhills on the FG was...bracing, shall we say.)
So, was it just me or is there something about the way a fixed gear's momentum pushes you thru the stroke that makes mashing up a hill easier? That's my hypothesis.
One variable: the club ride I rode geared was at a faster pace. But the hilly stretch was on the early outbound section of the "faster" ride where I was presumably fresher, while it was near the end of the "slower" fixed ride where I would presumably be tired.
Also worth noting that I am not by any means a strong climber.