When do toddlers start saying yes?

[Chickadee]

My 1.5 yr old daughter is very good at saying No. No to everything, even when she means yes and really wants what she's being offered. It's really funny, but I'm curious when she'll learn the word yes? We say it to her, to try to correct her, but just get another No in return.

[draught]

My 26 month old daughter still doesn't say "yes" very often - but she started saying "Okay" at about 18 months, and still says it all the time instead of "yes". She also says "good idea" instead of "yes". We all find ourselves saying "okay" all the time now as she says it so much it is contagious. If she wants to do something she will say "okay park, okay park, okay park" over and over until you either give in and go to the park or distract her with something else. Despite all of that though, she does still often say no when she means yes, so I am not sure when that stops.