Hi Maddy,
In my experience plus what others say the majority of people on a JY holiday are in their 50's & 60's so you'll fit in just perfect.
Saying that the age range is much larger than that, on the two trips I've done so far the ladies have varied from late 20's to 70's while the men 40's to 70's. Expect the ladies to outnumber the men, people say ts around 3 to 1 on average, the trips I did it was 11 to 7 (Americas Golden West) and 35 to 10 (Treasures of Andalucia) which sort of confirm that.
Mick