None.
I am 18 years old, and bought my 2000 Volvo S80 T6 in 2006 after my 16th birthday. I have not had a single problem with the car and it is a joy to drive!! Several other friends of mine also drive Volvo, and have not had a single problem.
This car is very comfortable. If you're looking to buy any Volvo - look no further!!
Good to hear some people haven't had too many problems with their cars. Make sure you look after it!