Yeah, it doesn't seem to make any sense, I agree.
The only logical explanation I can offer in terms of the actual story is that its not just his strength and sensory and super-crossword solving powers that evolve over time, but his ability to process solar energy as well.
What that means is that the reason he is not fully powered as a boy is not because he has had less time to process the suns energy, but that his body is actually physically not able to process it to the same potential as he can as an adult. His upper limit as a boy is vastly less than as a man.
So when he starts to recharge after the nuke, he does so faster than as a youth, and to a much greater upper limit.
Plus - he gets recharged at the fortress by machines which make the accelerate the process.
Of course the real reason is that folks don't want years of Superman stories where he works his way back up.