I don't know how aging affects the LED power draw, according to the manufacturer I shouldn't expect more than 7.5A. When measuring peak power output, I get only get ~6A total though.
4 pins are for earth with each (measured) having ~1.5A going through them at peak brightness. The fifth pin must bear the total load of the four other pins.
Having 5 pins is of course not a strict requirement, it's just the LED strip that has 5 connections.
Edit: I should have clarified that the 4 pins "leading to earth" are connected to mosfets controlled by PWM signals, so they aren't directly connected to earth. Each of the 4 pins carries a unique amount of current. Their total current is flowing through the fifth pin. Sorry for missing out on that detail in the original statement.
I haven't noticed any difference in brightness between the first and last LED. Since the power is sent via PWM controlled mosfets, splitting the power wouldn't work all that well since it inevitably have to connect the box (with the connectors) with the PWM circuitry.