There are two theories with Lavender. One is give them a short back and sides after flowering in summer (say, August), and the other is to do it in mid-spring, after the worst of the frosts have past, (say late april). I have a low lavender hedge 30 feet long, and I've tried them both over the years and they both work. The disadvantage of the summer one is that you don't enjoy the dried flowers sitting on the plants through the rest of the year. The advantage of the summer cut is that the bushes are much tidier for most of the year, and flower earlier and better the next year.

I trim my lavender pretty hard with electric hedge clippers to the size I want and I don't care what I'm cutting through, and the plants look brilliant and are the size I want. It doesn't seem to matter that I'm inevitably cutting into some old wood. But what you do need to do is ensure that you are not cutting hard back into old wood at a time of year when they will be (soon) exposed to frost - that does kill them.