I think this is the website for the provider the cruiselines use for the Dolphin swim. According to this website there is a Dolphin Encounter, Dolphin Swim Adventure, and a Dolphin Royal swim. Oh and a sea loin encounter. For all but the Dolphin Encounter the minimim age is 8 years old. The Dolphin Encouter is 1 years old and up. "Children under 8 years old are welcome to participate when also accompanied by a paying adult." In the encounter you are not swiming with the dolphin, but you are touching them and watching them do tricks for you. I hope this helps. If you book through the cruiseline they may have different age requirments I don't know.
http://www.swimwithdolphinsvallarta.com/