The second week of first grade, my 6-year-old son came home and told me, very seriously, "Mama, I have a girlfriend, and I love her."
I didn't laugh at him or tell him he is too young to have a girlfriend, and I didn't minimize his feelings. We had a very serious conversation about his girlfriend: what he likes about her, what they talk about at lunch, and what games they play on the playground at recess. I asked questions about her; some he knew the answers to, and some he didn't.
Nearly every day after that for some time, we talked about his girlfriend, and in every conversation, in some way, we talked about consent — what it means, what it looks like, and how I expect him to act.