Information on the age of a patient at disease onset, an important feature of complex diseases, is often collected in studies designed to map the disease genes. Penetrance-model-free methods, requiring no specification of penetrance functions, have been used extensively for detecting linkage and association between marker and disease loci. In this paper, we conduct an analytical study to examine the effects of incorporation of age at onset information on the power of two commonly used penetrance-model-free methods, the affected sib-pair (ASP) and transmission/disequilibrium tests (TDT). Assuming a Cox model with a major gene effect for the age at onset, we quantify analytically how age at onset affects the identity by descent (IBD) probabilities, the mean IBD values, and the expected numbers of alleles transmitted from heterozygous parents to affected children under various genetic models. We show that the power of the mean IBD test and the TDT can be greatly affected by the ages at onset of affected siblings or children used in the study. Generally, the most powerful test for ASPs is that based on affected sib pairs both having early disease onset and for TDT analyses is that based on trios with early-onset children. Naively combining affected sib pairs with different ages at onset or parent-children trios with different ages at onset of affected children can result in reduced power for detecting linkage or association. These results may be used to guide collection and analysis of sib pairs or families for diseases with variable age at onset.