Translation of ~のことだ?

Question

I was wondering how I should or could translate "no koto da" as in

僕が六歳だったときのことだ。?

I get the general meaning of the sentence, but I have no idea what the meaning of こと (koto) in this case is.

Answer 1

The Little Prince?

I can't give you a grammatical explanation but it's the difference between:

1) 僕が六歳だったとき...すごい絵を見た。
When I was six I saw an amazing picture ...
2) 僕が六歳だったときのことだ。...すごい絵を見た。
It was when I was six. I saw an amazing picture...

Unfortunately I'm having a hard time describing the difference between my English translations. I suppose in 2) 僕が六歳だったときのことだ is setting the scene for everything that comes afterwards. Whereas 1) is just a boring statement that the boy saw a picture when he was six.