NTA.
This situation sounds a lot like mine's right now, my brother is also the youngest and he is very much entitled because he was spoiled for being the "only" boy. I was very fortunate enough to receive a 20k car from my parents when I was a teenager and my sister received my dad's jeep. My brother is a teenager right now and currently at the age where he wants a car too. He told my parents he wanted a tesla, which my parents denied to. He threw a huge fit and demanded my parents to buy him one. My dad said a luxury car was not fit for a teen who just started driving and my brother didn't agree with that. My brother then went on saying that if my parents didn't want to buy him a tesla, they should get him a BMW or a Mercedes. My parents didn't agree and suggested that they could buy him a car that was worth 20k like mine or he could receive one of their old cars. He blew up and got so mad, he said that my parents shouldn't be treating him like how they treat my sister and I. He claimed that he was being treated unfairly and he deserved a fancy car. It's been an ongoing topic in my household and I think my dad is giving in to the tesla. I don't really have any issues if he gets a fancy car or not, but his attitude about the whole car is what ticks me off. He says he deserves it and that this is what my parents should be giving him. (sorry, this was sort of my rant/story)
I think it's amazing that you treat your kids all equally and kudos to your daughter for saving up her money and buying herself a new car. Your son is still a kid and I don't really know what to tell you to do since I don't know what to do with my brother. But you are definitely not the asshole.