I didn't put him there

I was cooking Nate's dinner last night when I turned around and saw this:

Cute, yes? My little man sitting in his high chair with the container of Cheerios we keep on the table.

Yeah, except I didn't put him in the high chair. He had climbed up on one of the dining room chairs, then onto the dining room table, crawled across the table, climbed into his high chair, and sat down. I know this because he had just done it 10 minutes earlier while Kate and I watched him.

(Note: the playpen behind Nate in the photo is now out of use. He climbs out - without harming himself in the slightest - every time we put him in there. Needless to say, taking a shower has become an adventure.)

So I'm thinking the high chair has met the end of its days with Nate. For a long time now, he's been standing up in it when he's done eating and trying to climb out of it, but we always grab him and lift him out before he actually has a chance to attempt it and either succeed or fall. But now that he can climb in it? I think we're done.

If you have a child, when did you transition from the high chair to a booster or regular seat? Nate can actually sit in our regular chairs and reach well enough to eat at the table but I still feel the need to strap him down or he'll just be all over... Yet at the same time I don't want to be a parent that feels the need to strap her child down. To eat a little more easily, though, I think he does need a booster... so maybe booster with straps while he learns to sit at the table nicely?

When do children learn to sit at the table nicely, anyway? Four? Five? Fifteen?