I have two Chihuahua/Jack Russell mixes, both are brothers from the same litter. They are 2 and a half years old. One of them is 15 lbs and the other is 12 lbs. I think it varies because their brothers and sisters are only 7-9 lbs. I always thought that my sister and I overfed them when they were puppies, but I just read about someone else's Chihuahua/Jack Russell mix is 5 yrs old at 25 lbs!
I have two brothers - same litter as well. One is 17 pounds and the other is 21 pounds (who should probably be at 19-20). They are 9 years old have been at these weights for several years now.
They stop growing at about 1 to 1 1/2 years old otherwise if they are still growing they are only getting fatter or skinnier not taller
I have a 15 month old girl who only wieghs 7lbs. However, she was the runt of the litter.