I am 35, and I am starting at ATP Jacksonville in July. So 35 is not too late to get started depending on how you look at everything…
My understanding is that 1500 hours qualify you to get your final certification, the ATP certification, nothing to do with the school, it is a certification. At that point you are legal to work for airlines, but the majors won’t hire you until you have spent some time at the regionals “paying dues” essentially. You could choose private or cooperate flying instead of regionals, but it is not as direct of a path to the major airlines. I am sure some more veteran people will add to this or correct me if needed, but I mainly wanted to say that you only live once. If it’s a dream, chase it. Good luck
Matthew is correct. After you build the required 1500hrs (most common route is the instruction) you’ll be eligible for your ATP license and can fly for an airline. While you technically could fly for a Major, that is the pinnacle of our profession and something that needs to be earned. The Regionals are suffering from a shortage of pilots and its there that most gain the experience and skills required to hopefully get hired by a Major in the future. Keep in mind nothing is guaranteed.
If pilots all had the choice to fly for the regionals or majors, almost nobody would chose to fly for the regionals. The vast majority of regional pilots are working to build their flight time and experience so that they can get hired by the majors.
I see that you just joined the forum yesterday, I would recommend you spend some time in the FAQ section of this website as there is a ton of great information there.