It would have to be a spacecraft which has a place in the manned compartments where a person of 50 to 70 kg could secret himself/herself well enough to avoid being spotted by the first person to walk by. The contraptions we currently put into space make a submarine appear luxuriously spacious by comparison. This teenager also has to get by security, and given the potential for shenanigans if a space-faring vessel were to fall into the wrong hands, that teenager will be passing by many signs with red letters reading USE OF DEADLY FORCE AUTHORIZED. Remember that nowadays, with far less at stake, you don't get through airport security unless you have a legitimate reason to board the plane. If we're talking a milieu with Trek levels of technology, nobody is stowing away without inside help; scanning the ship for unexpected passengers/packages will be SOP, and so this teenager will be caught. Pray that you're caught by nice people, who will only put you in the brig until the ship returns from where you snuck aboard. People who are not so nice may space you, and if they're really not very nice you could wind up as a human trafficking statistic; after all, there's no official record that you were an authorized passenger and once they've "corrected" the security footage, no proof that you were ever on board. Just buy a ticket.