i just came back from ibiza, me and two mates rented mopeds for two days and it was fine, you will be ok as long as you drive sensibly. We had 125's which were quite nippy (70mph) and they cost 29 euros a day (bout twenty quid). You just have to hold a full license, dont think there is an age limit for mopeds. I am 19 and it was fine so......
If you do decide to go for it, be careful i agree there are some stupid drivers..just drive defensively.