I think you're at about the right weight for your height to be well balanced...The lowest weight possible is around 175, as there are a few boxers that are in that weight class with the same height as you are. Antonio Tarver comes to mind, as you did mention that 25 as being an "old" age to start boxing, but Tarver turned pro at 28...I just finished watching FNF tonight and they had a HW prospect at 27 who had 10 fights under his belt already. There are more and more fighters out there who are up there in age that end up having long careers because they take care of their body in and out of the ring...
Going back to the weight thing, as I said the lowest weight is 175, the highest limit IMO is 195...I am around that weight at 6-3, but that's w/o training. If I was going to box, I'd probably be at 185...Playing around with 200 is not good if you plan on fighting because that could mean you could weigh above 200 (heavyweight minimal limit) which at this stage won't work...I know fighters that are at 225-235 that can't take a solid punch from those that are stable at 250...The last fighter that I can remember that was able to transition to the heavyweight ranks from the lower classes with much success was Holyfield, and that was just about when the bigger fighters came along (Bowe, Lewis, etc.) with skill that were able to take over the division...
Check out David Haye when he goes for a belt against Valuev to see the difference between those at our height (and that were at our weight) against these "super heavyweights" that are way over the 225-230 pound weight that most of todays fighters are at now...