there can be lot of reasons behind this i.e:
The most obvious reason why the laptop screen goes black can be a screensaver.
There might be some sort of problem in ur battery.
As u have said that ur laptop is older so i think ur hard life span is expired.
If a laptop is left idle for a long time, it sometimes slips into hibernate or standby mode automatically, in order to preserve power in the laptop battery.
Check out your video drivers.Maybe it is your video drivers that are malfunctioning.
These are some reasons.
Give me more information about it.