Home inspection should be able to tell how many layers the roof is, not sure about the age, maybe a guess or homeowners know. My house is 50+ years old and when I bought it it had 2 layers, with second layer only a few years old. Not sure what the code is there but here its no more than 2 layers.
As for buying old or newer. You could buy a brand new house and have lots of problems and have a 100+ house be perfectly fine, it all depends on how it was constructed and how it was taken care of I think. A good through home inspection is a must in either case. -- "When the power of love overcomes the love of power, the world will know peace." -Jimi Hendrix