I assume you are vaccinating with Heptavac P Plus?
You shouldn't vaccinate lambs younger than 3 weeks because it compromises the passive immunity they get from their mothers. Some folk don't vaccinate lambs until they have them gathered for shearing in June - I guess whatever suits your management system. I do ours when the youngest is about 4-5 weeks, so the oldest will be 8 weeks or so but we have small numbers, so gathering them isn't a major task

If the mother wasn't vaccinated herself, then she will not have passed any immunity on to the lambs, so in the same circumstances, I'd do them sooner rather than later.