Lilacs are the NH State flower, and at the end of May, the air is thick with the scent of them. They can live a long time, but as they age, they will start to flower less and less and may need some pruning. One thing to bear in mind is that if you prune at the wrong time, you will sacrifice the next crop of flowers, since the plant won’t have time to recover from the pruning AND create new flowers. So prune them just after they have finished blooming.
While looking around for some good lilac blooming diagrams, I found this article by Fine Gardening, which sums up the pruning process so well that, rather than re-invent the wheel, I am adding a link to it here.
Happy pruning – enjoy your renovated lilacs!