Development wise, in a purely physical sense, there is still a lot of growth from 18-20. in 2 years time, (which isn't the exact age gap, but whatever) Crosby could theoretically get stronger and taller. He should at least add more muscle mass, though him getting much taller or getting towards Ovechkin's current physical build will never occur.
The thing I'm confused about is a lot of people who are touting the age gap, and the physical development of an 18 year old versus a 20 year old, are missing the types of games each seperate player displays. Ovechkin's size and strength matter particularly more, because he is more physical. Certainly Crosby will benefit from adding more muscle, but his game is never going to be the bullish drive to the net Ovechkin's can be.
To respond to the people bringing up the jump in numbers from the 18 year old seasons to their second seasons in the NHL, numbers do not necessarily prove a point there. For some of the players listed, and others who make strides in their second seasons, most of it occurs just as much because their supporting cast improved a bit improved and their own 'experience' from having a year in the NHL. Growing in physical size and adding muscle certainly helps. But being able to do a few more crunches and pushups only helps so much. At some point in time, talent and situation take over. In Crosby's situation, is 10 more pounds really going to make that much of a difference? He still won't be 'large' by NHL standards. He won't be faster. He won't become a demon on the boards. I mean, what do you expect players to turn into Zdeno Chara's in 2 years of development? I just don't buy it. The rigors and experience (read: coaching/training) of the NHL are as much a result in the performance increase as any 10-15 pounds could be. It's not like NHL players are going to grow a third arm from years 18-20.
The irony of most of this discussion is Crosby supporters make it full well and knowing that they can say Crosby's 18 year old performance is clearly better without there being any recourse for Ovechkin. It is not Ovechkin's fault he was born late, or there was a lockout. I'm not saying he would be as good as he is now, or put up the same numbers. But there is no way to say he couldn't have at least shown a significant impact like he has had this year. He played extremely well in the RSL, decently in international tournaments, and was basically the same size he is now. If the rules had been like they are now, I think its a pretty safe bet to say he'd at least of challenged for the Calder.
What I don't get about all of this argument is why Crosby fans have to resort to defending him so adamantly on the lines of him being 'a lot younger'. Many people who are using that line of logic suggested he would be clearly better than Ovechkin prior to the season. I mean, obviously Crosby's numbers at 20 should be better than what they are now. He will have had a year and some change of experience in the NHL under his belt. I just don't get why so many have to fight tooth and nail to try and convince someone on the internet that your savior Crosby is 'disadvantaged' by his own age. He is doing a good enough job of making his own statements on the ice as it is. Both him and Ovechkin have interesting futures.