Are there any studies of epigenetic difference between twins during their lifespan?
I would think there have to be, though do you mean collecting samples on a regular basis and plotting out the difference at each point? Or do you simply mean the total accumulated change. If it is the latter, the answer is certainly "yes". You have probably seen the Nova documentary "Ghost in Your Genes" (The US, not BBC one). In it they show comparison of accumulated epigenetic change between a pair of identical twin Spanish women. I'm guessing you are more looking at measuring this change on a regular basis and looking for points of rapid divergence.... I don't know if this has been done for individuals, but it appears that it has been done for sample sets (see the same documentary). They discuss accumulated genetic change in younger twins vs older ones...