Any age... Typically under 25, but adults get a version, lada- latent autoimmune diabetes in the adult that acts and looks the same as youth version- require Insulin to stay alive.
Rare < 5. The loss or inability to make Insulin defines type1. It is very rare but reported in early infancy. I've seen 2 <2y in 30yr, most cases seem to cluster in 7-12yr age group. With this definition, we also see late teens or adulthood and can be confusing to some who only expect type 2 in adults.