This is not well-known among young people, but most elderly people do know this term. Know this is a slang though it doesn’t have much insulting. 若い[wakai] means young, and 燕[tsubame] means a swallow. So literally, it means a young swallow. But actually this means a young man who has an elder mistress. Mostly he is taken care of by her so nicely that he doesn’t have to work There’s another similar term, ヒモ[himo] which simply means a thread. It derives from connotations that a man forces a woman to work so hard that she gets skinny like a thread. But this doesn’t even necessarily mean he is younger than her.
I guess it could be a gigolo in English if I translate 若い燕. The reason why this term is well-known among elderly people is, that this was born in the Meiji period when most women used to be forced to work as a mistress so badly. So a woman who was the advocator of the liberation for women, happend to fall in love with a young painter. The fact that such an advocator dated a young man shouldn’t be in public…but it came out to the light so badly. Most women got upset and furious when they knew about the two. So the painter left her with a letter saying like, “a young swallow has to fly away to calm down the troubled pond…” This is how this term was born, and at that time, it got very popular.
Speaking of younger, there’s another term though it has nothing to do with a young swallow; 青田買い[aotagai]. 青田[aota] means a blue field literally, but this makes no sense. To begin with this, I must explain the difference between the two colors, blue and green. We Japanese say occasionally 青い[aoi "blue"] to describe something green such as a signal[青信号"aoshingou"], an apple[青りんご"aoringo"]. And also, in order to describe something immature, we even say 青い. I think this is the same thing in English. So, 青田 means a field which is not ready for havest yet. Even too early. 買い means purchase, buying. So basically, 青田買い means to hire ungraduated students even before they get ready to start job hunting. I said, basically. Yes, inside otaku circles, there’s another use. I think you can guess! 青田買い means to keep an immature boy or girl before they get grown-up in order to make for their own only. I think this term has a connection with ロリコン[rorikon] or ショタコン[shotakon]. If you don’t know these two terms, please check my previous post about ロリショタ.