I'm in my 30s too and just started PT a few months ago after a few peak helper runs.
I had my shoulders redone 17 years ago, have survived two near fatal car wrecks that ruined my back, my hands and left arm have nerve damage (they shake badly sometimes), and have spinal narrowing and blown discs.
I still out work the kids, care about my job, show up (on time) and bust ass to do anything that needs to be done.
UPS doesn't recognize the redeeming qualities of a hard worker or good worker so expect just to be as fast and strong as the youngins.
I seem to get along fine. Only issue is pay. I'm looking at driver by the time I'm 35 and get out by 55 to 60.
It's never too late, man. I'm still eyeing freight and receiving jobs that pay 50-150% more, though.