Infra red remote control very short range
The simplest, best-case scenario for you: There's dust on the receiver lens that's blocking some of the light from reaching the receiver.

Another possibility: The plastic on either the transmitter or receiver lens has yellowed over time. Most plastics will do this, especially if exposed to UV(?) light.

Another possibility: The LED in the transmitter has degraded over time. There are a number of mechanisms by which LEDs lose efficiency over time as you suspected.