PP - I think that's simply a misunderstanding along the lines of:
There is talk of finding a replacement for Tirdent, ergo, the submarines must be rather old.
The last Trident boat (VENGEANCE) was commissioned in 1999, IIRC. The first came into service in about 1993. The issue is one of beginning to study what (if anything) replaces Trident - which will be 'ageing' by the time this is necessary.