I'm doubtful of "senior" devs under the age of 35 (and certainly 25.) It's not to say it is impossible but it is unlikely. Being a senior anything requires more than just knowledge it requires maturity and experience. These are just hard to find when people are younger. Again that is not to say it is impossible, just that it is very unlikely. Regardless of age, older or younger, I would still be looking for a minimum of 10 years of actual work history.

I also am one of those developers that started early. I started programming when I was 12 and started my first software business when I was 17 and sold it when I was 19. However, getting an early start doesn't mean you attain the necessary skills any earlier.

No amount of technical knowledge would automatically qualify a dev as a senior regardless of age. Certainly knowledge about multiple technologies is required but a senior dev needs so very much more than that. Many many soft skills that take time to learn and develop can only be learned from working with people at multiple levels, in different environments over a long period of time. These are things people cannot learn from a book, in a classroom, or late at night hovered over a keyboard.

Senior devs need to know how to, negotiate, triage, manage expectations, mentor, coach, organize, and plan. They need to be excellent debuggers with intuition and insight at all levels; code, system, integration, and people. They need to have excellent communication skills, both in person and in writing. They need to know how to deliver bad news and the correct way to say "no". They need to know how to teach and learn and learn what they teach. They need patience and an understanding of what motivates people. Great senior devs have passion but little ego. They know and accept what they do not know and are curious, not threatened when someone knows something they don't. They have made mistakes themselves so they don't pass blame. More than just having all these skills, they have to have a desire to share this with their peers. senior devs are more than just technical resources, they are leaders.

There are many very good developers with excellent technical skills that don't have those skills yet. Some never will. The primary obstacle to those developers becoming excellent senior developers is often themselves thinking they have these skills when they haven't given themselves sufficient time to learn and practice them.