My lilacs are older and most of the flowering is up high (7 ft).
Pruning lilacs is limited, since cutting back partially will cause the branches to stop blooming for several years. To rejuvinate a tall lilac, remove 1/4 to 1/3 of the branches at the soil line each year. Choose the older branches first. The new growth will bloom in a few years on growth on the lower part of the shrub. The only other pruning that doesn't stop blooming is to remove the spent flowers after blooming each year. Broken or dead branches can be pruned back, but the resulting branch will not bloom for awhile.