Future Timmy is the future adult version of Timmy that appeared in the special episode Channel Chasers. Throughout most of the special, he is obscured by a mask, and his intentions are seemingly villainous, until he finally reveals his true identity and mission to his younger self after being cornered by him.
When he grows up, Timmy Turner becomes a strong, intelligent adult. There are two versions of this Timmy, one of them is from a grim, devastated timeline; while the other is from the good time line that was originally supposed to occur before magic interfered, which was restored in the end.
Both of these only appeared in Channel Chasers.
During Channel Chasers it was revealed that Vicky got her hands on a powerful magical TV remote. She used this remote to reach Dictator Week on the Biographical Channel, allowing her to raise an army to conquer the world. Twenty years later, Timmy Turner, now a toughened adult, disguised himself as one of Vicky's assassins to sneak in amongst forces and gain her trust. When Vicky captured A.J., she stole his time belt and sent the masked Timmy Turner back on a mission to the past, presumably to erase Timmy from history. There he was able to track down his younger self and convince him to stop Vicky rather than hide in television. Eventually, Vicky managed to damage adult Timmy's time belt, causing him to fade back to his time. Younger Timmy stopped Vicky, and this catastrophic time line was averted.
Bad future Adult Timmy wears a black suit with an ammo belt worn across the chest, and a pole worn on his back. He also has a five o'clock shadow. This uniform is also the same uniform worn by future Vicky's body guards. He is extremely strong, smart, and skilled at combat. However, he was no match for his younger self's magic, and was eventually captured and cornered by Timmy who unmasked him. When young Timmy learned his older selves intentions, they joined forces to stop Vicky. He was unfortunately unable to help his younger self the final battle, because Vicky destroyed his time belt, returning him to the future.
Adult Timmy is very serious. At first, he does not even attempt to reason with his younger self, just to capture him and bring him home, as well retrieving the remote. But he warms up to his younger self when he learns why Timmy ran away, and explains to his younger self that he remembers his parents being there for him all the time, such as on Christmas. The two Timmys become very close, until the tragic moment when Vicky zaps Adult Timmy's time belt and he is returned to the future. This sends young Timmy into a fit of anger as he vows to defeat Vicky, thus preventing the bad time line from ever happening.
After Timmy successfully averted the disaster he caused, he grows up to be a responsible, successful adult with two children. Its unknown who his wife is or if he is even still married, because his spouse does not appear in the brief period the good future was shown (it is possible that she could to be Tootie). He still lives in his childhood home in Dimmsdale, which has been restored to its former glory and now has a more futuristic look with tall buildings, flying cars, and such.
Although not as buff as his battle hardened bad future counterpart, adult Timmy is still noticeably well fit. He wears a white shirt with a pink tie, blue jeans and black shoes. His face is clean shaven. He also wears a watch which beeps when he needs to be at work.
This version of Timmy is more like how his parents were to him in his youth. Loving and caring, although somewhat dimwitted. He leaves his children, Tammy and Tommy, under the care of an evil babysitter, which he finds oddly familiar but not at all a problem. His children are miserable like he was, likely because their parents are always working, and they have Cosmo and Wanda as their fairy godparents. Timmy sees a picture he left in a time capsule of his ten year old self and his goldfishes (which look like normal fish in the photo) but smiles in memory of something, before heading off to work.
The mother of Timmy's children was never revealed, but has nonetheless been a hotly contested subject throughout the Fairly OddParents community. Timmy's children have both genetic and fashion hints from three different characters: Timmy, Trixie Tang, and Tootie. This was done intentionally by the creators to make this issue only resolvable in the fan's own minds.
After the release of the Fairly Odd Movie however, Timmy and Tootie were made an official couple in adulthood (at one point even married, although this could've been a gag). Assuming Tootie immediately had children, Tammy and Tommy could have still been born in time for the Channel Chasers ending to have believably taken place. Also, the official site for the Fairly Odd Movie held a character quiz, and one of Tootie's answers was that she would name her child "Tammy or Tommy". The creators have avoided talking about this issue directly in any way shape or form.