I found out my Dad was adopted when I was about 8yrs old, sometime around then. Can't 100% remember. But they did hold off telling me until I was of an age where I could understand not to bring it up infront of my Grandparents. That was their key reason for telling me later. I don't think they waited for a specific age but more a specific level of maturity, which is different for each kid.
I had a few questions - but this was the whole truth for my case - Grandma and Grandad couldn't have a baby, so they adopted.
When I asked well what happened to the other people - I was told - the lady couldn't take care of him but must have loved him very much to make sure he went to such lovely people. My Mum was very much like - sometimes we have to put the needs of others ahead of our own and your Grandparents are your Grandparents and they are Daddy's parents. There was some explanation on how we don't talk about it with my Grandparents because they raised him and they love him just like he was their own, so it would upset them to be asked lots of questions.
I was definitely old enough to grasp that as a concept and respect it. I never had a conversation with my Grandparents about it.
I think it helped that everyone in the scenario was painted as an incredibly nice person! Your adoption is different to my Dad's. My Dad's was a social services, he didn't know any of his birth family type of adoption.
So my advice would be to tell them but do so matter of fact and just allow them to ask questions. 16yrs and 10yrs is a different age group to when I found out but I do remember neither of my parents made a big thing out of it, and they told me in a way that wasn't emotional, which meant I was able to explore the topic and ask my own questions, rather than be 'told' alot of stuff.
Like I say, I was a bit younger than your kids, so I understand the reasons my parents took the approach they did. But it really wasn't a big deal.