That's true. He could just resume aging normally from that point as opposed to immediately advancing to the age he would be in relation to the years that he has existed. I may be getting superlore mixed up with vampires or mummies or something; It's all very confusing.
There is no reason in hell why a being with an artificially prolonged life would instantly, magically advance to their "true" age once the prolonging mechanism was interrupted. That's a particularly ludicrous fictional trope. Aging is not caused by the mere progression of the calendar. It's caused by gradual wear and tear on the body -- cells accumulating genetic errors and failing to replicate or work as well, joints and tendons wearing out from cumulative stress, toxins accumulating in the organs, DNA strands replicating so many times that their telomeres shorten to nothing and they can't replicate anymore. If there were a mechanism that halted or reversed all those processes to keep your body at peak health for centuries, then if that mechanism ceased to apply, you'd still be at peak health and it would take decades for that cumulative damage and decay to take hold.
As for the influence of the Sun on Superman, remember that it's not just our Sun, it's any yellow star. Assuming that means F or G stars inclusive, there are something like 40 billion of those in our galaxy, about 10 percent of the galaxy's stars (not counting brown dwarfs). So there would be plenty of worlds where Kal-El would have the power to be Superman. Of course, red stars outnumber yellow ones by about 9 to 1, so there'd be plenty more where he wouldn't.