Left: Twilight at around 1-2 months of age. I headcanon that while these ponies don't walk basically from birth like real life ones, they are able to kinda start stumbling around after their parents within a month or so. From then on, they age similarly to human toddlers, and won't start speaking for another 6 months at the very least. Unlike canon, I'm thinking that flight doesn't manifest until her tiny useless wings actually grow to size, though she'd still have some pretty powerful magical surges.
(Twilight would still be with her father here, and hasn't had her divinity bound by Juliet yet, hence the wings.)

Right: She's around 9-10 years of age here, shortly before gaining her Cutie Mark. Twilight got her Mark unusually young for a filly, as they are usually gained around the onset of puberty at ages 11-14. If she was still physically divine at this age, this is around the point where she would've stopped growing, and she would've had to figure out her destiny as the Stars to start growing again rather than her Mark incidentally appearing in a powerful, Rainboom-induced magic flare.