Oh, and to clear up the confusion for ages... Eren began and ended his training at age 15, it's slightly hidden on the wiki, but if you look at how old he is stated to have been when the titans attacked (age 10), as well as what year it was (year 845), then what year it says he finished training (850) and how long it said that he had to wait to join in between (five years), then the answer is right there