There is no age limit on travelling. One of the fundamental benefits of travelling is that it releases you from your home culture's preconceptions about how people should conduct their lives, hence any concerns about what one 'should' be doing at any one age have no place in the travellers mindset.
I can only really think of the east coast of Australia as an example of a popular backpacking location outside of Europe where the crowd is particularly young. Everywhere else - Asia, South America, Africa - you get the whole range of ages. Obviously certain locations will attract specific crowds, you don't get many 50-year-olds tubing in Vang Vieng for example, but there will always be a range of activities being enjoyed a range of age groups somewhere nearby.