That's not true. Most just charge a surcharge for under 25. When I was under 25 I bought a AAA membership which waved that surcharge as several major car rental companies.
Most of the truck rental places DGAF how old you are as long as you have a valid license. All the B2B ones basically expect that the person doing the rental won't be the one doing the driving and they're gonna slap a min-wage 18yo in the seat regardless.