Honestly, does it matter?

I've known 18 year olds I would trust with my life. And I've known 50 year olds I wouldn't trust to take out my trash.

I think its really a judgment issue-- not an age one (although I recognize the correlation). She's not going to age fast enough in one year that her judgment is likely to improve much... Don't you think?

How old is this AP?

This could be due to inexperience but it is more likely lack of common sense. Trust your instincts. We were in a similar position with a live-in nanny and gave a warning after one week, three weeks after that we had to fire her on the spot due to serious safety issues. You can try a warning but personally I'd contact the agency and let them know you'll probably need to replace her.

Common sense dictates that you listen to your gut. You don't risk your child's basic safety like this.

We have a new au pair (our 5th AP), and I am seeing what I think are small signs that her judgment is not great. I'd try to explain, but the issues would sound so nitpicky I'm sure I'd get blasted. Our job is primarily looking after one very active 3 year old (and sometimes his 6 year old brother). I'm having a small voice in the back of my head that senses she is not watching the 3 year old carefully enough. And for people who have spent any time around 3 year olds -- this kid stands out as a kid who REALLY NEEDS WATCHING.

Has anyone else been in this position? What did you do? How did it turn out?

She seems like a nice girl who is trying hard -- but I'm worried that this is a skill you can't teach. If you don't inherently know that watching him tightrope walk across the 6 foot high retaining wall at the park is a bad idea, I'm not sure I can teach those things.

any advice?