No, that's a widespread but false myth. The presence of prior generation wireless devices has never forced the whole network to use the old technology exclusively.
Your N-capable AP will use N when talking to N clients, and use G when talking to G clients.
However, the presence of prior generation wireless clients can slow down the modern stuff down a little, but not all the way down to the old technology's rates. This comes primarily from two things:
- New-generation stuff often has to operate in "legacy protection" mode to protect its new fast transmissions from getting stepped on by legacy devices that can't detect the new fast transmissions. These protection mechanisms cause a minor performance hit.
- Old-generation stuff uses lower signaling rates, and thus burns more airtime to transfer an equivalent amount of data.