As an adult and an individual I would have been inclined to say younger - because then you don't know what you're missing with regards to foods as you would have just never got used to having them in the first place and the pricking and the injecting would be second nature to you, the norm.
But then as a mother, and when I look at my two kids aged 3 and 6, I think I would give anything for them to have a normal childhood before having to deal with ANY of this stuff! I would want them to go to parties, eat what they want, fit in and not worry, not be ill or have to prick and inject themselves multiple times a day.
So on that note I say older. Older because, although it might be harder to break long held habits, you can deal with hard things better as an adult and no child should have to feel different or left out or restricted compared to others.
Diagnosed with GD in 2010, Completely disappeared postpartum. Re-diagnosed December 2012 with type 1.5 diabetes, age 26, BMI 23 currently controlled by only Metformin, 500mg twice a day.