– No. Never.
Backpacking around the world for an extended (indefinite = vagabond) with just a backpack can crudely be considered another form of travel and vacationing. With that said, there is no age limit of travel. The difference however is that backpacking is a far more deep, adventurous and enduring experience. So while age does not matter, moderate physical and mental health does apply.
Note to the kiddies: In your travels staying at hostels and guesthouses, you will run into “mature” or “senior” backpackers more often than you might expect. Talk to them. They often have some of the best travel stories and a unique perspective of world travel.