Man, don't feel embarrassed to buy Pokemon cards. I've gone up to the checkout with over $100 in Pokemon stuff and only that. Know how many funny looks I got? None. Not one. Know why? Cause it ain't their business. They don't know why I'm buying so much stuff. They have no clue it's for me. Their job is to sell it to me and put it in a bag for easy carry-out. If they want to tell people how some guy bought $100 worth of Pokemon cards, they aren't going to give you trouble to your face. They can get fired for that. If they do give you trouble for it, tell their manager, they'll sing a different tune I guarantee it. Who knows, you might get some free stuff out of it. So don't be ashamed. Buy your cards and be proud. Or at the very least, don't be ashamed.
I'm 21 years old and am a die-hard Pokemon fan. I'm never going to stop playing, either cards, or games, or whatever new game they might release down the road.
Age ain't nothing but a number. Keep that in mind. Pokemon isn't just for kids. There's a reason it's rated E for everyone, not E for kids under 12, or whatever. Everyone can enjoy Pokemon. The anime gives Pokemon a bad rap. Why play with stuff like Pikachu and Buneary when you can have mega monsters like green ears black whatever? Digimon has huge giant monsters too, that is obviously way cooler than Pickachoo. :/
Most Pokemon fans will agree that the anime is trash. It could be so much better. But I digress.
To summarize, don't think you have to hate Pokemon just because you're getting older. If your love for Pokemon does start to fade, it'll happen in due time.