At 26 you would be one of the youngest, I have been on trips where the age ranged from 20 up to 80, even at 45 I am often one of the youngest on the trip, but have never found age to be a problem, as long as you go along with a positive attitude and join in. I have been on 8 so far and have 2 more booked (Costa Rica next month and Vietnam for Christmas) this year and have always found everyone to be welcoming and friendly regardless of age and always had a great time, remember age is only a number.
Just pick a trip that you like the look of and go for it, I regret not giving these trips a go sooner.