I personally believe Enlightenment can be sought at any age, and worked upon. Although it is said that twenty-eight is the youngest age a person can achieve Enlightenment as before this time the required forces have not yet fully incarnated.
Throughout our lives we will experience (hopefully) a series of Spiritual Awakenings, all of which lead to our final goal of Enlightenment or Higher Consciousness. For most people the incarnation of negative astral energies will have been completed by around 28, and a solid ego structure formed. For those who do not follow any spiritual practice or meditation technique, their energy will stay much the same after this point.