That can be a very complicate affair :)
If they address you with "Du", it's pretty safe to answer back. Unless they are your teacher, or an older person commanding respect.
For people your age (and yes, I checked your profile pic) - you're pretty safe to use Du.
If you've been going with the formal Sie for a while, it can get pretty awkward. Normally the older person offers to say "Du".
My 75 year old mother has a new partner - he is still saying "Sie" to me - I address him with "Du" - 20 yrs of living abroad knocked the formalities out of me. It's become very odd now, because he can't snap out of it, and I refuse to get drawn into it. I offered, but of course it's not my place to offer. So we continue with our weird arrangement.
Books could be written about this topic. Just go with what feels right. Stiff and crotchety - definitely use "sie". If you travel and stay in hostels, go with "Du", you'll be fine.