YMMV. When I was <25, I got away with pulling prestige/adrenaline cars out of PC all the time. Probably did it >50 times and stopped a total of once.
Negotiating upgrades though, probably marginal. I never tried that myself. The few times I needed to directly reserve adrenaline/prestige at airport locations, it was probably around 75/25 on whether the counter assigned & allowed me to take adrenaline/prestige.