As has, already, been stated, &/or alluded to, ... weight is not a good indicator of age.
Some snakes, even though within the same species, grow longer/bigger than others. Plus, even if of the same age, etc. ... can have, vastly, different weights without being malnourished/underweight (and, of course, if malnourished/underweight).
Now, on to what I really intended on making a reply to
...
I assume he will stop growing eventually, he's been getting duller and duller so I will just act like he is middle aged
Actually, snakes continue to grow their entire lives.
They grow rapidly (&/or noticeably) the first year/s of their lives.
After true maturity (&/or mature size), growth just slows dramatically.