What is the ideal age (in weeks) to transplant outdoors?

I live in Michigan, Zone 5a. I was thinking of planting my seeds on April 1 then planting after the last frost in Michigan (usually at the end of May). The plants would be roughly 7-8 weeks old.

Sounds like perfect timing. As long as you are growing photo period strains, you can keep them indoors indefinitely under 16+ hrs of light. All they will do is grow larger and you should have reached the point where daylight will keep them vegging. They might go to flower early if you do not plant them in a southern exposure (max light). Flowering is usually triggered at 14 hours or less of light…The issue can be - they get so much light they will not flower until September and you may have monster size plants