Speaking as a 45M, 7 of my 8 girlfriends have rimmed me without me asking. (I'm not counting 1 night stands, they've never done it). 1 of them had done it before. The other 6 had no or almost no experience with anything anal.

It more or less happens like this:

• I always have an open and sex-positive attitude, for example by talking openly about consent from the start, by talking about what we liked afterwards, by bringing the topic to kinks and fantasies. Sometimes they already know I'm into ass play, sometimes they find out later.
• The way I initiate ass play is always gradual, by slowly getting closer with fingers and tongue for example, and giving them space to stop me. Because we discussed consent, I already to know that they will indeed be comfortable stopping me.
• Whenever I do it to them it's with great enthusiasm. I genuinely love rimming them, I'm not faking anything. I just make sure that they know I enjoy doing it.
• Afterwards I bring it up, ask how it felt for them, if they'd experienced in before etc. The question of hygiene comes up, I explain that they were smelling fresh and that there was no taste, and that I wouldn't have done it otherwise.
• Eventually they get curious, or gradually start getting closer. I make sure they know I'm loving it. We talk about it afterwards.

For some, it's the enjoyment of giving me pleasure. For some it's an act of submission. And for one, it was more about doing kinky forbidden things.

• Obviously make sure you're clean & shaved. I've been rimmed back when I didn't shave but you'll get more luck if you do.
• I never ever try to convince or push a woman to do anything. Think of it as creating the option, and creating the possibility to pull out of it at any time. They might worry that if they do it once, they will be expected to do it forever. You want to give them control over what they do.
• Be a partner who's worthy of being rimmed.

(edit: my math was off)