I do not know about tough Love, but yes, God uses both Justice and Compassion. In fact this whole Scheme is a Process of Creation, Preservation and Transformation. Still, let us see it this way: if the child strives to touch the hot coal, and you prevent him by slapping, it is still Love and not 'tough' Love.
The ages you mention are too old. It will mean that you have probably failed if you need to discipline an 18 or 25 year old. Start from birth, as by seven or eight, it will be difficult, and 10 or 11 almost impossible, let alone 18. Much Love.