When I talked to my 3 yr old and almost 5 yr old I told them that most ppl in the world are nice and kind but that sometimes there are bad ppl that we need to be Careful around. I talked about how they might try and give you candy but that we should politely say no and find a teacher or mom or dad if that happens. I also talked about if someone grabs your or touches you and you don’t know them to scream.
Sad reality we live in and I’d rather have my kid over react to someone then be taken or hurt by someone

I think now is the right time. A lot of nightmares come from kids walking off with strangers at the mall, park or any public place without hesitation. It’s not going to teach them to be afraid of people. It’s teaching your child to be safe.

Maybe around school age so that they will understand better. Because you don’t want them to be scared of other people