I'm not a Boomer, but I think it's unfair to stereotype only Boomers as giving unsolicited advice. I think maybe it just seems more obnoxious because the parenting style comes from a very different generation.
But there are parents in their 20s giving other parents unsolicited advice. About what? Some of the things I've heard: "I know the right diet for your baby. It's X." "Never buy processed baby food - make your own." "There's no reason babies can't be raised Vegan, and in fact it's better for them." "Never set your baby down in the first year - you need to 'wear' them on your body at all times like the Yanomamo tribe does in the Amazon." "Letting your child play unsupervised in the back yard is child abuse."