This is an under-appreciated cost of working at startups. It's fun to be the big fish in the pond, but it means that you have less access to resources for this sort of thing.
You've identified something that you think is high value to you, and should reasonably be supplied by your employer as part of your job -- access to a mentor. This is something to consider when looking at jobs.
Startups don't have to comprise merely junior employees. Sure, the senior employees rarely have a skill-for-skill match to your profile. Startups can't afford that kind of redundancy.
But, any competent startup should have a good balance of senior 'ship steerers & NO men' alongside dynamic starry eyed juniors. For product & work-life mentorship, you don't need a technical peer as your mentor. So any senior leader in your startup will do.
For technical mentorship, I do think it is a bad idea to join a startup as your first company. Kind of leaves you without reference. Some avenues for technical mentorship that have worked for people around me are:
1. Universities sometimes have alumni mentorship programs
2. If you worked in a lab in undergrad, your professor or his ex-PhDs are great references
3. Friend of a friend, friend of family. 2 degrees of separation casts a wide net that often catches 1 person who is a good match.
4. If you can find a good technical mentor who just doesn't have enough time, then getting access to their wikis or favorite blogs also works. Sometimes the hardest part is understanding workflows and curating the right information. If someone has already laid it out for you to read, then it's about separating the wheat from the chaff.
I totally get what you’re saying though. It can be a situation of the blind leading the blind.
I work at a bigcorp and have had great mentors, and now that I'm an old fart, I try my best to mentor others. It's a huge plus to have access to a large community of talented engineers