I’m not certain it has to do with age. I can guarantee you I’m older than you. I may change my mind in future books, but I have a problem with her treating the MC that way because it tends to cause confidence issues in children and, later, adults, when a parent treats them as though they are creatures in need of constant protection. Especially after being an absentee parent for most of their lives. And treating an adult that way is even worse, and grating, IMO.
Edited to add: Also, treating a child and adult this way typically has one of two effects: you either end up with someone who is afraid of their own shadow or you end up with someone who is so determined to prove the parent wrong that they recklessly endanger themselves and insist on doing everything alone to prove a point. Neither case is good.
I do sympathize with Rebecca, to an extent. It wasn’t her fault that she had to lie for so long. It isn’t the lies that I see as a problem, though, it’s treating the MC like a small child instead of a capable adult who has managed on their own for a long time.
Unfortunately, I saw these examples as mollycoddling. The first one, in particular, was just to get the MC off track enough to accept UB’s presence (if the team was taking over, certain personalities would not have taken it well at all and been like “do it yourself then and leave me alone”). For the “I don’t need protection” one, again, it seemed like little more than pacifying (a mother telling her pre-schooler that, of course they can tie their own shoes, but I’m offering so why not let me help?).
Anyway, I get that Rebecca, as a mother, loves her child despite being an absentee parent. But she needs to get that the MC is not a child anymore and is capable. By the end of book 1, I think UB sees the MC as more competent than Rebecca does. I do believe a lot of Rebecca’s attitude can be explained with guilt she feels over lying and for not being there, but that’s her problem to deal with, not the MC’s problem, so she should figure out how to deal with it while still giving the MC credit for being a capable adult instead of a moron who has to be pacified and can’t tie their own shoes.