Population aging is exacerbating the loss of competences in the workforce and simultaneously young people neither in employment nor in education and training (NEETs) are struggling to be reengaged in employment. These issues, which are deemed priorities for the European policy agenda, could be addressed by triggering active aging dimensions, valuing and exploiting older adult entrepreneurs' knowledge for enhancing youngsters' entrepreneurial attitudes, through mentoring. This paper reports the results of a study based on an intergenerational learning program, carried out in 2018 in Germany, Italy and Slovenia. The study was aimed at developing and testing one training on mentoring addressing 41 older adult entrepreneurs (55 and over), and two intergenerational learning trainings aiming at boosting entrepreneurial competences of 33 NEETs (aged 18-29). The impact of the program on older adult entrepreneurs and NEETs was assessed through a pre and post-evaluation using qualitative and quantitative tools. Findings at country level were treated as national case-studies and then the latter were compared by considering them as a multiple embedded case-study. Results indicated that, to different extent in the study countries, mentors learned and enhanced mentoring competences, e.g. active listening and the capability of orienting, improved well-being and self-esteem, social inclusion and active aging attitude. Moreover, NEETs acquired entrepreneurial and socio-relational competences by benefiting from the full exploitation of mentors' know-how and the trust relationship with them. Companies, trade unions, educational and voluntary organizations should cooperate to adopt intergenerational learning programs as good practices for older adults and NEETs' lifelong learning promotion.