Many eCheck accepting poker sites will require you to be either over the age of 18 or over the age of 21 to be able to legally play at their respective poker sites, depending of course on what the legal age to gamble is in your country of residence.
eCheck is accepted at a lot of different poker sites, and as such you will never have any difficulties funding your poker site account using eCheck as long as you are old enough to sign up and then play at those poker sites.