For starters, it's Totally Spies. I think that sets the expectations for writing quality more than any other factor. Secondly, they were kinda-sorta written like adults in the first place. For most of the series the three of them lived with each other as housemates instead of with their parents, and they work an incredibly dangerous job which their lives are focused around. Aside from high school, they are effectively emancipated. That's long before they explicitly age up the characters for college. Aging them up again wouldn't change much.
Contrast this with Kim Possible, a roughly contemporary show with a similar theme, where it's almost central that she's school age and not independent from her nuclear family.
If Winx Club Season 8 is anything to go by, a reboot is more likely to make them younger. According to the producers of Winx Club, you can make cartoons for babies, you can make cartoons for adults, but the market in between is rougher than it used to be. Both of these shows are in an unenviable position where a lot of their established fans are adults now, but there's no way in hell they can transition to an adult audience. On the other hand Totally Spies might have an easier time staying the course because it's not a show about magical fairy girls.
I wouldn't know since I never actually watched Totally Spies. Maybe some scenes here and there but not a full episode. I definitely never watched Winx Club.