You are not properly charging your battery and it will never come to full capacity charging at only 100mah's. The minimum charge rate for a 2000mah rated capacity Nimh such as yours is the C/10 rate or 200mah and 14 to 16 hours initially. A common mistake is when changing up to higher capacity battery or newer chemistries and not obtaining the proper chargers.

As far as is it getting to old, well I gernerally replace my Nimh's after five or six years.

John