It's not a case of it getting older, its a case of u putting more miles on it.
U could have a 20yr old car with 25,000 miles on it, thats serviced properly, maintained, cleaned every week etc etc and it'd run fine.
or u could have a 20yr old car with 150,000 miles on it, thats serviced properly, maintained, cleaned every week etc etc and it'll start playing up cos it needs new bits.
Thats why I like to keep with newer cars, cos "usually, not always" they are less hassle.
over 100,000 miles on a PETROL engine and things can start going wrong (not always, but can)
*and b4 u say it, yes even a 500mile car can get engine probs, but its alot more rare*