You sound like me lol, I never had the courage to invite a girl out ever in real life, only thru online where I could say everything I wanted and didn't stammer, and I'm 41, so just be yourself and go for it, if you delay for too long I think you are just going to overthink it even more, one thing, we are so much more than our stammer, if she says yes, great if she says no, It is not because of your stammer, no nice person would ever do that, and I think you wouldn't want to spend time wtith a person that doesn't care about you. Take care! :)