As it has been said, age and understanding depends a lot on honesty. I'm not saying lie to your children, but when they're young they may not understand.
For example, about this whole COVID business. My 5 year old is understanding it a lot more than my 2 year old. My 2 year old has no idea what COVID even is, thinks life is going on as normal, and it makes me a little jeaous sometimes lol. So, there's no use sitting down my 2 year old and even trying to explain because it's over their little noggin. My 5 year old blames it all on the "corona" and while she might not have it right all the time, she gets why we can't just go out to the park or why all the people in the store are wearing masks or why people are keeping distance from each other.
Honesty is great when the children will understand it.
Sometimes I get why parents don't tell their children everything. They are not required, but sometimes parents don't want to freak their children out. I remember when I was younger. I'm not sure how old I was, but my parents pulled up to their bank to check the balance and they were low on money. They were like, "We have how much!? What did we spend it on?!" Like kind of freaking out. They didn't usually act that way so we all were so worried. My mom agreed to cook more meals at home instead of go out because it was getting too expensive and costing too much money for all of us. My sister was the one that was freaking out for days. She was like, "We could be homeless! We could be living on the streets! I may not have a bed to sleep in!" Like...she took what my parents reacted about and said over the top in her mind. She was of course exaggerating, but she took the situation as she saw it. It didn't do her any good, but everything was all right and we were never homeless.
There are many factors other than honesty that affect why children don't just run to mom and dad to talk about issues. I remember my parents hiding the stressful issues, how much my dad made at work, and things like that as those were private things to them. My parents were open to us about a lot of things such as death in the family and such...some hard topics, too. There were still things I didn't want to run to Mom or Dad to because I feared being in trouble. I thought, "Oh no, if I told them, I would be in such trouble!" From what I understand, even if these were not present in my situation, some parents that are overly-controlling, overly-discipline, over-react, etc. sometimes cause a child not to return to their parents for help. Sometimes that stroke of teenage independence and the, "I got this!" attitude when they don't exactly got it is a factor.
There are so many things, we can't blame the honesty thing alone.