So here's the rub: Knees aren't for life, unfortunately: Knees wear badly - due to knocks and bumps. So look after them: protect them from impact by wearing knee pads.
Internal knee pads are very discrete, and most worktrousers come with knee pad pockets these days. (Knee pad pockets are a standard size thankfully).
Integrated with kneepad trousers, specially designed knee pad pockets, a benefit - they never come loose or move out of position.
The IMPACTO knee pads tend to be most popular.
External knee pads are especially handy for temporary use (and by being external they also protect your trousers/coverall from wearing through, when on your knees).