Why is there a stigma to elderly developers applying for internships?

As an elderly but intellectually spritely developer (51) looking to shift focus to front-end development, I've been looking at various remote internships... only to find I'm getting figuratively strange looks as I'm not a recent graduate or product of a bootcamp. Assuming that it's not entirely down to me as a candidate (of course it could be!), and given that there's at least another 15 years of work left in me, is there some unspoken code that I'm unaware of?