I know what Tim Hancock is going to say
My oldest (turned four in December) thinks it's time to move up to the top bunk. His younger brother (two last October) is moving into the bottom bunk from a toddler bed.
My four year old is very advanced and I have no doubts that he understands the rules and we just spent the last 15 minutes practicing the ladder and explaining "one firm handhold at all times" rule. He navigates the ladder with ease and is deliberate. He understands no climbing in socks and he has never fallen out of his lower bed.
The top bunk has a rail except for the ladder area at his feet and a similar gap near his bed that we have blockaded.
Bed mfgs claim 5-6 years old is the line but liability certainly plays a big role there. One mfg says as soon as they can traverse the ladder safely they are good to go.
His comment just now of "Daddy, I'm so proud of myself for being ready for the top bunk." doesn't make the decision any easier.
What are your thoughts?