Age 54 here (having returned to taekwondo after a decades-long break). I'm always trying to get more adults to take class at the school I attend. Our adult/teen class is mostly teens. During some months, I'll be lucky and have 4-5 adults taking class with us, but more lately it's just been me or maybe me and one other adult in a gaggle of teens. I don't mind the embarrassment though, largely because I'm doing this with my 7 year old son (but when he and I take class together of course, we attend the All Belts class, not the Adults/Teen class - then I'm often the only adult).
For me the most embarrassing parts weren't class, they were the color-belt tests/ceremonies with my son. Often it would be me and maybe one other adult in a sea of 40 children taking the test or attending the ceremony. You can't help but stand out when you're a short stubby old man sitting on the floor in a sea of children, testing for (way back then) 5th gup. That having been said, so many parents have come up to me after and said, "I think it's so cool that you're doing this with your child. I could never do that of course, I'm too (whatever excuse: old, non-athletic, easily embarrassed, non-limber, whatever...feh, excuses!) but I think it's so cool that you do." So I don't focus on the embarrassment part, I focus on the fact that people view me...the adult...as a role model. Even a lot of the children will confess to me that they wish their parents would take class with them. So many of the little chatterboxes want to just talk to me all the time too; I don't know, I think it makes the children feel like they're doing something cooler because they see it's something a grownup would do too, so they want to connect with me more?
My thinking is that as a 25 year old taking classes with mostly teens, you're sending a message to the teens -- and even the children -- in your school that it's not uncool to keep doing taekwondo as an adult. They don't have to quit when they turn 17, they can keep going into adulthood. Or looking at it another way, leaders almost always stand out...and standing out is almost always embarrassing. That's one reason why most people shy away from being leaders: they don't like being the person who stands up first, or speaks up first...that and the fact that leadership is usually more work too.
Personally, I think it's a badge of honor that you're there by yourself as an adult. That having been said...I sure do like it when more adults show up to my class! (And as an aside, I wish I would have kept going in taekwondo long ago, rather than taking a break. Originally I was diverted by a Ph.D., but still, looking back...could I have not found time for both? I wonder.)