Usually you need to be 18 to drive a car in Europe, although local variations apply.
Mostly those under 18 who have a driving license can only drive in their own country and often only under supervision of specific people.
To rent a car you usually have to be older, 21 or even 25, and have to have had your license for some time (usually 2 years or longer.)
If you and your friends are all under 18, forget a road trip. If some of you are over 18, you can travel by train or bus or use a tour. Or drive a car you own as long as you have the proper insurance.
If all are under 18, your best bet is a tour aimed at people under 18 but it is possible to travel alone (singly or as a small group) as long as you have parental permission and can proof that.
Most hotels in Europe keep a minimum age of 18, for at least one in each group, sometimes for all in the group. Hostels sometimes have a lower age limit but you will need to check beforehand to be sure they do accept your group, sometimes there is an age limit because you need to get in through a bar or some such.