I don't think the US has reached it's peak just yet. So we will see the USA still be the superpower till at least 2100. The US might last till 2300 who knows, there has never been a superpower with the strength of the US. All those who say the US will fall or not be a superpower are just upset that the US can be so powerful and so young.
It's like this scenario: A man works at a company for 30yrs and has done many great things for the company. In those 30yrs he never worked for someone younger so it is a shock for him when the company hires a new boss a position he suspected of getting and the company chooses a new employee that has had only three years experience and he is in his 20s still unlike the 50yr old.
The US is a power unlike any other because we don't have those old traditions which hold us back like the others.