John Connor was a different age in each Terminator movie he appeared in. The first movie in the science fiction franchise - 1984's The Terminator, directed by James Cameron - famously stars Arnold Schwarzenegger as the Terminator, who is a cyborg assassin sent back in time from 2029 to 1984 to kill Sarah Connor, played by Linda Hamilton. This is because Sarah's future son - whose father is the time-traveling Kyle Reese - will apparently one day become a "savior" against cyborgs in the post-apocalyptic future, leading a resistance to defeat the machines. Her son is the now-beloved character John Connor.
The first Terminator movie saw enormous success upon its release. In fact, it topped the U.S. box office for two weeks. Moreover, it boosted the careers of both Cameron as a director and Schwarzenegger as a leading man. And, of course, it spawned an entire franchise. The Terminator franchise has, over the years, come to encompass numerous sequels in addition to a television series, comic books, spin-off novels, and video games, among other things.
The character John Connor doesn't actually appear in 1984's The Terminator - in this movie, he is not yet born, and is only referred to as the future son of Sarah Connor. John first shows up in the 1991 sequel Terminator 2: Judgment Day, or T2, which is often considered the best installment of the franchise. In the character's first appearance, John is famously played by actor Edward Furlong as a child, and very briefly by Michael Edwards as an adult. In this movie, as a child, John is meant to be a 10-year-old juvenile delinquent, and he fends off Terminator assassination attempts.
In the next movie, 2003's Terminator 3: Rise of the Machines, John is portrayed by Nick Stahl. This movie is set a decade after T2, so John is a 20 year old who has been living off the grid. In the fourth installment of the franchise, 2009's Terminator Salvation, John is played by Christian Bale. This movie is set in 2018 - in a different timeline - so John is older at 33 years old, and is battle-worn. 2015's Terminator Genisys, meanwhile, sees John played by Jason Clarke. In this movie, John is supposed to be 47 years old; his character is said to have lived up to 44 and time travelled back to 2014. This entrant in the franchise severely alters the past films' events. In 2019's Terminator: Dark Fate, meant to be an alternate sequel to T2, John isn't really in the movie, but a CGI version of actor Edward Furlong from T2 briefly appears at 13 years old. This movie is set only three years after T2.
In the parallel universe TV series Terminator: The Sarah Connor Chronicles, which aired only from 2008-2009, John is played by Thomas Dekker and is supposed to be only 15 years old at the start of the show. He turns 16 in the season 1 finale, and 17 during season 2. John's character does not appear in either the 2009 or 2015 web series based on the movies.
Of course, there are some discrepancies with John Connor's age across the Terminator movies, mostly due to timeline errors. In T2, John's year of birth is clearly stated to be 1985, and that movie takes place in 1985. In the next sequel, though, John states he was 13 years old during the events of T2. This was a major plot hole. There were also discrepancies with Sarah Connor's ages - she was technically supposed to be only 19 years old in the first movie, but clearly appears to be in her mid-to-late 20s. However, because of the time travel elements of the movie - which include parallel timelines - the filmmakers were able to write off most of these errors to best serve the story.