Sugar did not stop aging because of her Devil Fruit. That is a lie she told the Donquixote Family. The real reason that Sugar looks the exact same as she did ten years ago is because Sugar was never human to begin with.
For Sugar is in fact a Pacifista. Vegapunk gained knowledge ahead of time that Doflamingo was planning on re-conquering his family's old kingdom of Dressrosa, and just like how Doflamingo had a long-time undercover mole in the Marines (Vergo), Sugar was created ten years ago by Dr. Vegapunk and was specifically given a higher artificial intelligence for the sole purpose of infiltrating the Donquixote pirates without arousing Doflamingo's suspicion.
My reasoning for this is that Vegapunk, knowing that having a Warlord (who's openly hostile against the government by stealing the world noble's loot) in control of a member-nation of the government would lead to a dangerous situation, and so created Sugar to monitor/stay informed of Doflamingo's actions.
Vegapunk was also responsible for giving Sugar her Hobby-Hobby fruit, because not only does Vegapunk probably have access to unique, high-level ability fruits like that through his research, but also because Vegapunk is the only person who possesses the knowledge of giving devil fruit powers to inanimate objects and beings like sentient robots.
Evidence of foreshadowing for this theory is that, ever since Sugar's initial appearance, including the flashback set ten years ago when Doflamingo first took over, Sugar has always been seen wearing a hooded robe with bear-like ears, and lately a hooded robe with bear-like ears with a small royal crown on top. Remember that all Pacifista models seen in the series thus far have ALL been "Kumas" (Japanese word for Bears). So, Oda could be cleverly hinting her connection to Vegapunk and the Pacifistas. She is the Princess of the Kumas, or Princess of the Pacifistas, because she has demonstrated the highest level of artificial "Human" intelligence so far, higher than the standard Pacifista models (aside from Bartholomew Kuma himself, of course).
Additionally, Sugar always appears to be fairly indifferent and unsociable to other members of the Donquixote Family, like Trebol (who she always tells to go die.) This could be because, as a Pacifista, her true loyalty/allegiance is to Vegapunk, and she has no sort of emotional attachment to the pirates she's infiltrated and deceiving.
Also, it seems highly unlikely that Vegapunk, who has been mentioned multiple times to have a scientific IQ about 500 years ahead of the rest of mankind (to the point where he is drawing up schematics for items/weapons where the technology needed to create them doesn't exist yet) would be completely oblivious and unaware that his old colleague Caesar (whom he knows is shady and mischievous based upon the Punk Hazard Incident) is in collaboration with Doflamingo, one of the most renowned of all the Warlords. It's equally unlikely that as an expert in all things devil-fruit related, he would have no knowledge of Doflamingo's huge black-market Smile Zoan fruit shenanigans with Kaido.
It would not only be a good, unique twist to the plot, but also serve to further hype up the cunning and intelligence of Vegapunk, who (through Sugar) has been secretly keeping tabs on everything that Doflamingo has done and has always been one step ahead of him, outwitting Doflamingo like a chess-master and pulling his strings, pun intended. Vegapunk could, like Fujitora, desire to see the Warlord system abolished, and would use the information he has gained from Sugar as a perfect reason to bump Doflamingo off. It would also be refreshing to see a Pacifista that isn't a gargantuan clunk-head, but instead is small, compact, and a dangerously intelligent threat.
I even thought of a awesome quote to go along with my theory lol.
(that Vegapunk says to Doflamingo over the Den-Den Mushi)
"You sit on your throne, the comfortable king of the toybox. What do you see when you gaze out the window? Your defeated subjects. The misfortunate, hapless puppets, shambling about. And You, their masterful puppeteer, standing behind them, laughing. But Doffy, what do you see when you gaze into the mirror? Do you ever notice the puppeteer standing behind you? Whose laughing behind your back, "Joker"?!"