Basing this purely off when Andrew appeared in Equestria; he arrived on April 1 when the comm opened and his application states that he came from two days after he was "born." It's not the slightest bit clear in canon what time of year his creation took place; I suppose you could make an argument for the original air date of Journey's End but whatever, I'm going with March because counting from his arrival in Equestria is going to give the most accurate count of his actual age.