Lifetime Risk is the risk of developing a given medical condition sometime during one's life. Average lifetime risk is calculated for a population group (such as Caucasians of European ancestry); your own personal lifetime risk may be calculated by applying your odds/risk ratios (based on your genotypes, assuming high heritability for the condition in question) to the average for the population you belong to. Incidentally, average lifetime risk for a population should specify which birth year/cohorts it applies to.
Often, however, population statistics are a bit hard to come by, and furthermore, the more interesting "personal genomics" statistic might be residual lifetime risk in light of your own genome and your current age ... but there isn't enough medical and epidemiological data to actually calculate this yet for any complex disease, despite what the marketing departments of various genetic testing companies might imply.
For Caucasian populations, a sampling of average lifetime risk for various medical conditions listed in SNPedia follows: