I am 53 years old and also have problems keeping up at the fiddle jams. I was told that the trick is to not play all the notes #(if you are learning from a book) but pick a simple basic melody. In otherwords, its not necessary to pick all those #running scales and arrpeggios. I now put in a few tremolos, fills,and licks into the music as I chord with the rhythm section (guitar/bass).
You don't have to play the song note for note or play all the time, also #become part of the background to the music, as rhythm and backup, and add a little spice instead. In our fiddle jams, no one does a break, only a kick offbefore the whole gruop starts in. So learn a few kick offs (and endings) to start (or finish) the song if you're called on. Mostly the one who chooses the song #does the kick off in our group. YMMV at other jams.
Las Vegas, NV
"Every day brings a chance for you to draw in a breath, kick off your shoes and play your mandolin."