By Stacey
Date 24.07.03 10:31 UTC
Mentally - never :-)
Physically - they are fully grown in terms of height and overall size somewhere between 6-8 months, but they will fill out a bit afterwards.
A Jack Russell terrier may be a mixed breed, or at least a mix of Jack Russells of varying physically characteristics, so ultimate size and conformation depends on the mix. Parson Russell terriers - the long legged type - are the registered breed in the UK.
Stacey