Q:

Is John Connor's age inconsistent in T2 and T3?

According to all the information I can find regarding Terminator 2:  Judgment Day, John Connor was 10 years old at the time the movie takes place:

John makes his first true appearance in Terminator 2: Judgment Day which takes place in 1995, played by Edward Furlong. He was a 10-year-old juvenile delinquent living with foster parents...

This makes sense, because he was conceived in 1984, and Terminator 2 takes place in 1995;  thus, he would have to be 10 or 11 years old.
But in Terminator 3:  Rise of the Machines, we hear this exchange between Kate Brewster and John:

KATE: You're John Connor. I'm Kate Brewster. We went to West Hills Junior High together.
JOHN: Jesus.
KATE: What happened to you, John? Middle of eighth grade, you just disappeared. And there was that thing about your foster parents?

In the U.S., eighth grade students are almost always between 12 and 14 years old - I was a bit younger than most of my classmates, because I was born in November, but even I was 13 when I graduated from eighth grade.  I have never heard of a 10 year old being in eighth grade, ever.  Most 10 year olds are in fourth or fifth grade.
John isn't a normal boy, either - he spent much of his childhood wandering around with his mother, blowing things up and learning how to use machine guns and stuff.  I can't imagine he went to school very often, if at all, for most of that time.  If anything, he should probably have been left back several years in school.  He certainly shouldn't have been sent into eighth grade two years early.
Is this an inconsistency, or is there some way to explain it?

A:

John is ten years old in T2. You can see the information briefly on his police record. It states his year of birth is 1985, and it states his age is 10, making the movie set in 1995. So from this information, and Sarah's doc stating she is 29, Sarah would have been about 19 in The Terminator. T3 gets it all wrong. John would not have been in 8th grade, and Sarah's tombstone should not say she was born in 1959. 

A:

You mentioned the exchange between Brewster and John, but perhaps you missed the opening narration by John Conner in T3 where he specifically states he was 13 during the events of T2. 
So yes, T3 delivered a huge plot hole. 
However, to be fair, the ages mentioned in T2 did not match up well to the characters. John and his friend looked closer to the 13-14. Sarah Conner did not look 29, and she certainly did not look and was not portrayed as 19 in The Terminator.
In "The Terminator", While Sarah's age is never given, it is clearly obvious in the plot that Sarah Conner is not intended to be 19 but more accurately somewhere mid-to-late 20s:

19, while technically legal adult, is still young and immature. They'd have a closer attachment to their parents. Sarah was already an established adult living on her own, with a roommate who also looked mid-to-late 20s. There was no mention of college and Sarah's relationship with her mother was reduced to brief chats on the phone. 
The narrative paints Sarah as having bad luck in terms of a career and love life. Sarah's waitress job is implied as a dead-end job, yet it would be perfectly suitable for a 19-year-old girl. It is implied Sarah has struggles with men and is at the verge of being an "old maid," but a 19-year-old wouldn't be concerned with settling down, they'd be just beginning their dating life.
The Tech Noir allowed Sarah in despite California at the time one had to be 21 to enter a bar.
During the police station scene, the police and psychiatrist would naturally want to notify Sarah's parents if they discover a 19-year-old was kidnapped by who they think is a madman.
Kyle Reese, assuming he's in his late 20s, would seem all the more creepy for kidnapping a 19-year-old girl, eventually having sex and impregnating her. Most 19-year-olds haven't even moved out of their parents house yet. There's the argument that Kyle Reese could be around the same age, which given his status as an experienced military veteran, is not likely.
Michael Biehn was 29 and Hamilton was 28 during filming, and they both looked it. Actors can play younger, but I don't believe this is the case here. The movie intended them to be somewhere in their mid-20s. At best, the movie pegs Sarah to be about 25-ish.

I believe the T2 reference to Sarah being 29 was written by the other script writer and it eluded James Cameron, or he simply changed his mind when writing T2. 
Referencing Sarah as 35 in T2 would have fit better. So yes, while T3 directly contradicts T2's ages, it more accurately depicts what their ages should have been in those films. Sarah being born in 1959, making her 25 in the first film, makes more sense. In the similar manner, T2's ages also contradict T1.

