I have been telling MILFs the following:
me: "have you ever heard of the term milf?"
her: (she thinks a little): "yes"
me: "YOU are a milf"

this is after a short, casual, non-sexual conversation. like when a milf is walking her dog, I will approach her and start talking about the dog, then after like 2 minutes, I say that line.

before I did that, I thought I would get slapped. But most of the milfs take it as a compliment. most appear flattered and say "thank you"

However, I suspect they feel uncomfortable because after that, they walk away. maybe its my fault because I dont know how to take it further. After the "thank you" what should I do?

or maybe they are taken aback by the sudden change in mood. like one second I am being this harmless guy and the next second I am telling her she is a milf?

or is this all wrong and I should stop telling milfs they are milfs?

to sum it all up, how do I tell a milf I am interested in her?

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I have not failed 10,000 times. I've just found 10,000 ways that won't work. - Edison

The time to tell a MILF she is a MILF is when you're having pillow talk. Of course by then she already knows you feel this way so it's pointless to call her this.
Yes by all means if you are going to call her a milf (i wouldn't do this) come with a sexy vibe and demeanor (come with this demeanor in any event unless you just want to be buddies and pals with the women you are approaching).
"You seem cool we should hang out let me take your number."

ocean is right no offense OP, but this is some of the dumbest shit I've read. Yes she walks away because you made her feel uncomfortable, and is trying to exit politely. I've been dating a girl for 4 months that I cold approached on the street with a confident fun loving bad ass demeanor. I didn't need to use any stupid lines to express sexual interest it was all in the vibe/energy I was projecting.

I'll give you a scenario you approach like that start off good then creep her out with that line and blow a possible good chance.

I approach with the same approach I just told you, and I end up dating the girl. I can honestly say that is most likely how it would play out because I'm speaking from experience.

MILF = Mom I'd Like To Fuck

You're effectively telling a woman after 2 mins that you'd like to fuck them.

That's a really bad pickup line (no offense)... I'd stay away from it.

If it's not working, why use it at all (anywhere in your repertoire?)

Go into qualification...

After saying your line. Ask her if she would ever date someone younger...would you have a problem going out with a younger guy...etc. Your immediately moving her mind on to another subject...

technically your frame/mood/vibe is probably changing after you make the statement and your looking for her reaction. Don't hold your frame/vibe and let her feel your strength.

You can also watch "Good looking loser" videos and how he approaches pick up. Could be very similar to your own.

I've dated girls much older than me. I like them because there's less bs and more straight forward. And I wouldn't have 'dated' them if I didn't know and game.