It's time...

1. It's time the baby were /was/ in bed.
2. Isn't it time we started off?
3. I think it is about time you started to behave like a grown-up man. /or just like a grown up.
4. The teacher said that the bell had gone and it was time the lesson had begone. / begun. I would omit had
5. He looked at the clock and thought it was high time he had gone home. / or went home
6. It was already spring and time /omit time/ the birds had returned.

unclear   it was spring and the birds had returned  (ie the birds are back

  it was spring and time the birds had returned  (ie the birds are not back, they're late

7. I was meaning to tell you it was time we had forgotten /we forgot/ our old quarrels.

unclear again, sorry

I was meaning to tell you (now) that it was /is/ time we forgot.... (correct is we forget, but common usage is forgot

I was meaning to tell you (yesterday) that it was time we had forgotten...

8. Isn't it time all our quarrels were forgotten?

poka