Maybe it's the way you're approaching the volunteer aspect of the job itself. You say you do the painting, maintenance, plumbing, electrical, etc yourself. It could be the park staff is trying to encourage you to tell or show the younger workers how to do this work, i.e. supervise them, rather than doing the work yourself.
When I was working some of my experienced co-workers felt they should do the work themselves rather then mentoring the younger workers because they could do it faster and therefore cheaper. What they didn't understand is if there were multiple projects needing work that they could do they were often the bottleneck on getting things done. Also by not training younger workers they were affecting the future growth of the company, and lastly if they couldn't pass on the knowledge of the jobs they did to other workers they stunted their own career growth opportunities - they couldn't be promoted to newer jobs because there was nobody left to replace them.
As someone else pointed out, working with younger workers and mentoring them is a real opportunity to pass on the knowledge and wisdom you've accumulated over the years. If you can pass this on to others than your work will continue long after you're gone and that's a nice legacy to leave.
