Lilacs flower on old wood. They definitely benefit from cutting the oldest branches out every once and a while. I too have heard that cutting old flowers to prevent seeds, is supposed to be of benefit, though I haven't done it.
not necessarily Mike. If you remove one third in a hedge cutting or shearing fashion, then yes, however if you selectively remove one third of the oldest stems to the base of the bush after flowering, like forsythia or mockorange, then you will be fine and dandy.
this is my hedge, its about 7 feet high and 30-40 feet long. didnt take 1/3, just trying to square it up a little. good idea mike i think i will do half. now back to the first question, where do i cut? seems like the buds at the base of the seed stems my be next years flowers?
Your thinking about fruit trees that have fruit spurs which flower year after year on the same plant part. Lilacs are more like spirea or some other multi-stemmed bushes, they flower only on the new growth. That's why you prune them right after flowering, never before, so you don't cut any of next years flowers.
That's why lilac flowers are way the heck up there where you can't reach them. LOL! It's also why apple trees are so showy, the flowers are all over the tree, inside and out.