@Galestorm
Why are you interested? Is it because you’re thinking that for the story’s sake she’s too old to be a baby sitter? Or because you’re worried she’s too young for the fantasy you have about her? Or because you think she’s too young to be a princess of her own domain? Or because of some tagging question?
What’s the context?
For myself, I think she’s old enough to be a baby sitter but not too old to be helping out the Sparkle family and sitting with a foal that she likes. And whatever age you want her to be in your fantasies is fine - that’s your business not anyone else’s. And she seems capable of handling the tasks and responsibilities that she has accepted, so I don’t think her age matters much. And for tagging on the site, if she’d drawn as an alicorn just tag her as Foalcon if she’s drawn “young”.
Other than that, I really don’t think there’s any point in talking about their age. It’s not something that I think we have any context for, or any access to, other than that certain models are used for drawing her in the show, and sometimes she’s shown doing one thing and sometimes she’s shown doing other things, and she’s whatever age is necessary and useful for the narrative.
For me it’s kind of like asking how long Sugar Cube Corners has stood, and whether it’s been remodeled in the last 5 years. Because we don’t have any way to judge “what a year is” in the show, other than via things like the Winter Wrapup and Hearths Warming Eve, and acknowledging that we don’t see the ones they don’t show us, who knows?
If this is just purely speculation and I’ve taken it too seriously, sorry … but as best as I can this is what I think about Cadence’s age:
She’s older than Twilight. She’s younger than Celestia.