|Old Man Dexter|
|Family||Old Gal Dee Dee (sister)|
|Occupation||King of Dextopia|
|First Appearance||Old Man Dexter|
|Voiced By||Christine Cavanaugh|
Old Man Dexter is almost nothing like any of his past selves, instead he is a senile and very forgetful old man with a jolly and often confused disposition. He can barely remember anyone's names, not even the names of his past selves, often calling them "Billy" instead. He also doesn't remember much of his past, even forgetting how he saved the future. Like Dexter, he detests Dee Dee and was willing to destroy her after remembering how she ruined his dream of saving the future. He also barely remembers Mandark, but continues to hold a deep grudge towards his former rival, exchanging petty insults with him when they were reunited. Despite this, he has no issues with letting Dee Dee's elderly self, Old Gal Dee Dee live with him or letting Mandark continuing to live, although the latter is forced to live out his days as an exhibition in his museum as punishment for his crimes. Despite his intense senility, he is still intelligent enough to build complex devices and teletronically manifest objects, abilities which he shares with his subjects who respect and revere him as a wise and benevolent leader, but in truth he mostly has more wisdom to share than knowledge considering how far gone his mind is.
Dexter first became his elderly self after using his Hydro-Cosmetic Accelerator to age himself enough so he could watch late night television without his parents objecting. However, due to Dee Dee's involvement, the device aged him way too much, turning him into Old Man Dexter, who complained constantly, had numerous health problems and suffered from forgetful senility, often calling people "Billy". Lucky for him, his parents and Dee Dee were oblivious to his true identity, instead believing him to be Grandpa. Dexter, however had no objections when it was finally time to watch late night television, but much to his misfortune, he ended up falling asleep due to his advanced age making him unable to stay up all night. Eventually Dexter was able to undo his aged transformation.
Old Man Dexter (along with Old Gal Dee Dee) makes a cameo appearance in this episode where Dexter imagines himself stuck with Dee Dee forever if the Chinese finger trap couldn't be removed.
Old Man Dexter seems to rule the world in the future where he makes everything perfect because of his science. He is very old and weary now so he is confused and not fully functional. He is forgetful and is usually having problems for instance he is always mistakenly calling people Billy. Although he can't do much, he was seen having the ability to do lots of inventing and work in Ego Trip when he helped his past selves defeat the evil Mandarks.
Old Man Dexter is seen still living out his days comfortably in his rocking chair until he is interrupted by his sister, Old Gal Dee Dee who proceeds to reminisce about the good old days. Dexter then recounts all his heroic deeds and amazing adventures through space, but Dee Dee correctly reminds him that most of his recounts of his past heroism are just exaggerations of the times they played together, much to his annoyance at being corrected. Eventually he goes into a long-winded tale that leaves him so tired he falls asleep in his rocking chair. His sister then happily puts his blanket over him and let's her dear brother have a good night's sleep.
- In the episode "Old Man Dexter", he is constantly mistaken for his grandfather, suggesting a similarity.
- This was later confirmed in the episode "The Grand Daddy of All Inventions", where it is revealed that Old Man Dexter really is the spitting image of his grandfather. Both even have the same voice actor Christine Cavanaugh,
- He made a cameo in the 20th episode of I Am Weasel.