What age do babies start to understand 'no'?

[beanstalk]

My DD is 8.5 months and is quite boisterous around other babies. She will pull hair, pat them hard on the head, poke them in the eye, stuff like that! She is just excited by seeing other little people and wants to say hello and explore I think, but obviously I can't let her run riot at baby groups
I firmly pull her away and say no when she gets too rough, but she obviously has no concept of potentially hurting others. I don't want to stop her playing with other babies but one or two mums have got quite funny about her being too rough. Others (thank god!) say 'don't worry, she's just being sociable, they can take it!', which is really how I would feel if it was someone else's DC with my DD.
Ooh, going off the point into politics now! Really I want to know, is there anything else I could be doing and what age should I reasonably expect her to understand that you don't poke other people in the eye!

[insywinsyspider]

ds is 16mo now but I used to take his hand and say 'gently' and show him how before he got too rough ie putting my hand over his and stroking, we've done this for the neighbours cat too and its worked really well, like you say they are just being socialable guess all you can do is show her how to do it in a more gentle way
HTH

[olivo]

beanstalk - i think my dd understood no at about 10 mo. i used to say no if she was doingsomething dangerous and remove her and she used to lie down on the floor and cry .
she definitely knows no now at 12 mo but still carries on doing things!! i still have to show her how to be gentle with babies etc, not her fault, she's just quite boisterous and over exuberant!!

[FrannyandZooey]

Just gently remove her if she is being rough and say "play gently" or similar

she may soon begin to understand that you react unpleasantly when she does certain things, but she isn't capable of controlling her impulses yet

[sugar34plum]

all mine understand they just ignore it!

[beanstalk]

Thanks for the replies, I like your suggestion insy, to actually show her. Think my problem was that I knew that she wouldn't necessarily understand words - although I thought she should understand tone by now she has shown no sign of paying attention to my tone! Lol olivo and sugar, think my LO may grow to be very similar!
Yes Starlight I am sure you are right about their personalities. There's a difference though, isn't there, between doing something that is dangerous or will actually hurt others, and just exuberant play? That's where the politics of it gets tricky because as you say I am more tolerant of it because my DD is boisterous.