The ability to produce high-quality semen in sufficient quantity is a crucial trait of artificial insemination (AI) boars in the pig industry. Therefore, the selection process of young boars at AI centers, apart from their genetic value, should specifically take into account the quality of collected semen. The aim of the current study was to analyze factors influencing semen quality in boars ranging from 4.5 to 16.8 months of age and to demonstrate the impact of these factors on selection of AI boars. The present multi-factorial analysis includes a complete standard spermatology data set of 4611 semen samples from 3633 young boars of five different breeds. Data were obtained from one reference laboratory during the years 2002–2012. Approximately one-half of the boars were excluded from use for AI due to below-threshold values of semen quality. In this group of excluded boars was a noticeably high number of boars less than 8 months of age. Boar age and breed were identified as significant (P < 0.001) factors influencing semen quality. The influence of season was marginal. Therefore, a minimum age is proposed for young boars to qualify for selection, or alternatively, age-related semen quality thresholds in young boars should be established.