"Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross." Phil 2:5-8
In one sense, you can say he aged because he became part of our 4 dimensional world - 3 dimensions of space and 1 of time.
If you are referring to the body deteriorating, we should remember that he was in his 30s when he died, so not really old and frail.
However, I think the answer to your question is that when he took on human form he also took on our frailities - which I think perhaps is a different way of saying what you said - "He willed it", (or possibly it was the Father's will which He was obedient to). He became subject to hunger (Matt 4:2, Matt 21:8), He cried (Luke 19:41) and of course He was subject to death.
Afterall, it wouldn't have achieved much if he stayed a baby, would it?
And I think He did this also so He can be our High Priest:
Since then we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast our confession. For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin. Hebrews 4:14-15
