From what I have read is that in general the older the nettle the tougher the leaves. So they get less edible because the are harder to chew. That being said I am collecting nettle for the first time this year myself
However, they are suppose to be great for drying for teas or other uses dry. The main hazard I know of is watching out for heavy metals in the soil. Since nettle is great at accumulating iron it can also accumulate any heavy metals like lead or arsenic from the soil. So be cautious in urban areas or around former waste sites.