Need to ask to date younger????

My bf of 4 years is 20 years younger than me. In fact my oldest son is 3 months older than him but we have a good solid relationship. Anyway his mother has always been funny acting with me and doesn’t really seem like she likes me. I can understand she doesn’t want her son with a woman old enough to be his mother but he’s a grown man and can decide for himself. When I spoke with her, she says she feels like I should’ve talked to her before I started dating him and asked her if it was okay. He was 23 at the time so idk why she’s holding this against me. Shouldn’t a guy that be able to make up his own mind?