Hi Sue,
I've done 5 trips with JY and in my opinion 58 is about the perfect age you'll be somewhere near the middle of the group. Thee majority of travellers are in there 50's & 60's but there are a good few either side.
From what i've seen ladies can be anything from late 20's to 80 while men tend to start later at about 40 to 80.
Like you i'm a seasoned single traveller but do enjoy going with a group, made some great friends and have 2 more trips booked.
Mick (aged 54).