Really depends on their condition when purchased, but 15+ year old cars is pushing it. Not worth buying imo, but I guess as long as they're allowed people will buy them. I think the taxes are higher on really old cars?
There are people who simply can't afford or don't want to put any more than a few thousand eur into a car. The other thing is that you can buy a 10-year-old car that is in good condition, and drive it as long as its lasts.. which can be many many years. (My first car was about 20 years old when I got it for free, and my current car was more than 10 years old when I bought it -- now about 13 years, and still feels almost like brand new to me :P)
Taxes are higher depending on emissions, but it's rarely enough to offset the purchase price. For example, if you buy a stinky 90s car that emits 200gCO2/100km, tax would be around 300 eur/year today. A new car that emits 120g/100km is going to be somewhere around 150 eur.
Also, first cars. The usual advice for new drivers is "get a cheap used car and drive it for a while, instead of dumping (equivalent of) a couple thousand EUR into a newer (used) one, and then crashing it into a gate".
