Pixiv

I have a few tags I use for age regression:

"幼児化" (Infantification)

"乳児化" (Also infantification)

"若返り" (Rejuvenation)

"ロリ化" (Lolification)

"幼女化" ("Young girl-ification")

"赤ちゃん化"

"赤ん坊化"

"赤子化" (All three meaning "babification", I guess)

Some of these terms don't yield that many results on Pixiv itself, but they're still worth remembering. I also use all of them in my general searches via Google, so I can find more AR; that's notably how I found, directly or not, most of the AR finds I mentioned here.