Clipping doesn't 'ruin' their coat or make it grow in heavier, those are both myths.
With age, some horses tend not to shed out as well or thoroughly. It can also be an early indicator of Cushing's disease or another thyroid disorder.
As far as the impact on their coat color, clipping more often, before the coat gets long, will help minimize the effect. Pigment tends to migrate to the end of the hair shaft, so with a long coat, you get the maximum color change, usually to that weird gun metal gray color.
However, if it makes your older horse more comfortable and allows him to stay in work, I'd say it's well worth it. I have had some older guys and some ponies with heavy coats that didn't shed out well that I ended up clipping year round - they were a lot happier clipped.