with seedlings I like to wait until they have a bunch of leaves (some people plant them when they have the first set of true leaves). The only way I can get lavender to thrive at my place is to plant it in raised beds (about 10 inches above the normal soil level) and in somewhat poor soil that is half mixed with gravel. And I only water them if they wilt, which they never do.

trianglejohn is right about the soil; raised beds and/or sandy soil are the key to getting lavender to survive our wet winters and springs. The soil where I live out here east of Raleigh is sandy loam/dirty sand so I don't have to worry as much about raised beds. But when I lived in PA and gardened in clay, I planted true lavender in a 2 ft high raised bed and it did pretty well.

I keep mine in a large pot. It's on the front porch in full sun but out of the rain. I will water it thoroughly about once a month in hot weather and almost never if it's not hot.

The only one I've had in the ground sulked until we had a drought, and then it florished in amended clay.