They’ve established–without really stating it outright–that Diana ages very quickly. I mean, if each season takes roughly a year, she was born slightly after the middle of season 3 and yet showed up at the end of season four as a kid who looked to be around three years old. At that rate of aging, looking like maybe a nine or ten-year-old a year later doesn’t seem all that unreasonable.
This is actually a trope used a lot in order to get characters introduced into the plot as babies to an age where they can actively participate in events. It’s called a Plot-Relevant Age-Up.

