Welcome! And congrats on following a life dream .

Judging by body composition and length, and considering you say he wasn't taken care of well (I'm assuming this includes feeding), and by the fade of his pattern I would put him at about 3 or 4 - Although he is about the size of a one or two year old ball python.

But judging age by size is difficult and very hard to do so I don't expect to be right haha