If you put a multimeter on the battery and then crank the engine, the voltage on a good battery will drop to about 10v during cranking.
The plates in the battery fur up (sulphate) over time and that increases the internal resistance which means that the cranking voltage drops.
On an old battery, it will drop to well below 10v and the cranking will be slow and may not start the bike.
With a decent quality battery that is kept on a battery maintainer the upper life expectancy is about 8-9 years.
With a budget battery that is never on a maintainer the upper limit is about 5 years.
If you have a decent battery maintainer, remove the battery from the bike and leave it plugged into the maintainer for a few days.
The good ones can de-sulphate the plates to a degree and will recover the battery somewhat, but don't expect miracles.