According to our sages of blessed memory Moshiach will be well immersed in the Torah, so how old then will Moshiach be when he is revealed.
As Maimonides states, "If there arises a ruler from the family of David, immersed in the Torah and its commandments like David his ancestor, following both the Written and Oral Torah, who leads Israel back to the Torah, strengthening the observance of its laws and fighting God's battles, then we may assume that he is the Messiah. If he is further successful in rebuilding the Temple on its original site and gathering the dispersed of Israel, then his identity as Messiah is a certainty."