I don't know all the details about morning glories. But many flowers
don't bloom until the right conditions are forthcomming. I would assume that they require so many hours of sunlight before they will bloom so I would expect to see the blooms start around June 22nd or there abouts.
It's like zinnias, if you plant them inside 2 weeks before the last frost they will get pretty big before they flower
. But if you plant them in August they will produce blooms before they are any size at all. It has to do with the hours of light they get not how old they are.