Just opened an old skool sativa this weekend that is just over 3 years old. It was average but ok.
I havent smoked that strain in over a couple of years and barely smoked at all in the past 18 months though so hard to give an objective view regarding smoothness, etc... Flavour wasn't there but I find that with hydro there is a drop off of that across all grows. I dont think that one had a whole lot going on flavour wise when fresh.
Having said that I opened an 18 month old jar a few weeks ago, different strains and the results were great. I dont put that down so much to the age of the bud but more so to the quality and characteristics of the strain.
So it can be difficult to tell but if maintained like fine wine a few years no problem