Hi George,
I think Wensleydale is typically sold as a young, crumbly cheddar type, aged 1 or 2 months (4 to 8 weeks). It should still have a fresh young taste to it, and be crumbly. You can age it out, and the flavour will lose that young aspect and mellow. I would be surprised if it "goes bad" and it may well develope some unique characteristics as you take it out to a year or more. I've made Wensleydale twice, but the first I added sage to and the second aged out to 5 months. The first melted really well, the 2nd had some issues during aging, but ended up with a nice cheddar flavour, smooth and creamy paste.
I've only made Derby once, and it was using my own make procedure. I'm aging it out to see how it does, so I don't know. I believe Derby can be aged and it developes its own characteristics.
I would just make a bunch of them, and age them to different targets and decide when you like it best. Sometimes a cheese is nice young, or quite old, but the middle period ends up being less good than others (teenage cheese, perhaps?) Anyway, this means it may not be worth the wait to age a cheese six months but take it out to a year or 18 months and you're happy again. Fun stuff.
- Jeff