We get quite a bit of condensation on our skylight which I think is about 15 years old.

There is a very dark line around the frame where the glass meets the frame which looks like the onset of rot.

The worst is that the condensation is freezing in this current cold snap and then thawing out during the day causing water to run down the walls.

yes, i believe it is past its best

Erm, yes. It’s leaking water and rotting. That’s pretty terminal.

In pretty much the same way as:

Is my car past it’s best?

It’s on fire, and the engine just fell out of it.

I don’t think its leaking, just condensation which is freezing then thawing.

I’m sure it will hold out until the summer when roofers are a bit less busy.

your velux should be double glazed so shouldnt have condensation on the inside. if it has, then the glazings failed which when combined with the rot means its probably past its optimum performance. time for a new one methinks. try some roofers now. its often a pretty slow time for them.