Depends what they are interested in. If they like barhopping they go barhopping, if they like sport they play even it's night, if they like online games they also play until dawn.

Depends on where you're at in the country. In the States, weekday evenings are typically boring unless if you live in a big city. Everyone's evenings are different, but my fun evenings usually work out like this; dinner, drinks, head to a friend's crashpad, out for more drinks and maybe some light tapas, night in a club until the wee hours of the morning, a post-club snack, and then recovery time. On slow nights I'll just grab a book, experiment with some new music or randomly browse the web. 

Many activities:

Young people live in the city usually: Online, Play games,...others study