It all depends on two things:-
- What is the law in your country
- How much you want to stay on good terms with this person
In the UK I believe it is illegal to state the age of a vehicle is younger than it really is. He would also be in breach of at least one consumer law (possibly more).
You can ask for compensation or return the bike and ask for your money back.
Problem is, unless he turns out to be a decent guy, you will have to get legal on his ass.