The age-structure of a population, and the distribution of sexual behavior according to age, are significant factors determining the spread of the AIDS epidemic. The threshold conditions for age-structured models account for life-history information, and thus differ significantly from their age-independent counterparts. We examine the threshold conditions for four general age-structured models of the spread of HIV in a homosexual population: three with random partner selection and one with biased partner selection. We consider both discrete and continuous risk groups, and the duration of infection. Susceptibility and infectiousness are treated separately, and the infectivity varies with duration of infection. Through specific examples, we examine the sensitivity of the threshold conditions to the population age-structure and the shape of the infectivity profile. The effects of each are of the same order of magnitude.