Short-form video sharing platforms (SVSPs) have seen a significant surge in older adult content creators in recent years. This emerging trend adds evidence to challenge the conventional perception of older adults as later technology adopters and passive online recipients. To investigate the reason behind the trend, we conducted semi-structured interviews with 13 older adult content creators on two of the most popular SVSPs in China, DouYin and KuaiShou. We found that our participants were initially attracted to SVSPs because of perceived ease of participation and the enjoyment they found. However, what kept them engaged was the attention and support they received there. SVSPs offered a low-barrier and equitable platform through their near-automatic use and relatively equal opportunities for recommendations, allowing everyone to reach audiences. Motivated by their passion for performance and the viewers' support, our participants actively acquired new skills for better performance and became more deeply involved on the platforms. Based on the findings, we reflect on how SVSPs' technical affordances support older adults in the transition from lurkers to contributors. We advocate for participation equity and supportive environments to promote more inclusive social media platforms.