He's only just realised that he can, and it's simply because his legs have reached the optimum length for couch ascent. His preference tends to be the dangerous north face, without crampons or ropes, which would be safe enough if he didn't bounce around giggling the entire time.
Of course, after the ascent comes the descent. His preferred method of reaching base camp - otherwise known as the pile of hard and potentially-hazardous toys he carefully gathered before starting his climb - is to wriggle off the couch, head-first. This usually results in me having a great view of a small posterior and a pair of legs kicking like frog hopped up on espresso, as his body disappears vertically towards the wooden floor.
Yes, my son loves to climb - and he seems to do it just to terrify me. I have a firmly-held view that children only learn to move around just so they can contribute to their parents' chances of having a cardiac arrest.
The staircase tends to be a child's first foray into the world of extreme sports. It is, of course, done without fanfare. Ideally, a parent should be unaware that the child is even thinking of climbing until said child is fully at risk. Then it's funnier for the little shrieking baboon.
The moment of revelation for a parent tends to be carefully chosen. Usually, something awkward - perhaps even precious and fragile - is being carried at the instant the parent becomes aware of their toddler crawling grimly up the stairs, somehow looking vulnerable and smug at the same time. The sound of screaming is better when punctuated by the sound of crashing and breaking.
Many people fit complex wooden contraptions to their staircases, to stop their child from climbing. These tend to result in a slight alteration in their cliffhanger tales, usually with added guilt for the parent who left the gate open. Also, the little demons will eventually learn to open the stairgate, and this is kept as a lovely surprise for Mum or Dad.
The best solution: live in a bungalow. Or arrange for a builder to remove the stairs. You can always get a rope ladder.
Escape to Victory
We watch our children very carefully, don't we? We never let them out of our sight. We are responsible, caring individuals who can be trusted to keep them safe.
Wait a minute - where did the little nightmare go now? I only turned my back for a minute!
They love to escape. To a small child, an open door is the same as a carefully-dug tunnel running 200 yards under the barbed wire. Mum and Dad are the Goons and they're asleep in the watchtower.
There is no solution to this problem. No one can glare directly at children 24 hours a day, although often we feel we are. Sooner or later, the spotlight will be turned off and Big X's little brother will start crawling for the perimeter. All we can do is hope we can catch him before he reaches the train to Switzerland.
Then we put him in the cooler - and he starts planning his next escape.
Stop hitting that baseball off the wall!