Four's mom looks like she's only 10 years older than him

With Four's receding hairline, it's obvious he's in his 30's (which he is in real life).

But Naomi Watts (although in real life is 16 years older and could have been his mom as a teenager, getting pregnant when she was 15) looks much younger than her age, and looks to be in her early to mid 40's at most.

When she was first introduced as his mom earlier in the series, I was completely in awe, as I didn't think she was more than 10 years older than him on screen.

Why not choose an older actress (or make Watts look older) so that you don't think you're looking at Four's older sister every time she's on screen?