Why would they need to be in their 20s?
They can do a Children's bible and start with that, and then when their vocab is a bit more advanced, go on to the full Bible.
I was in my 30s when I read the full Bible for myself in English. I think people can handle the New Testament a bit easier than the Old Testament since it's much shorter. But they won't get the full picture if they only stick with the New Testament. The New Testament is totally dependent and referencing what was in the Old.
it is suggested for the most clarity to the next generation.