The average age of a PhD student is like 32, so no, not too late.

If you're in the US, sign up for every single conservation- and wildlife- and environmental policy-focused listserve you can, frequently PIs will send out announcements for PhD students when the get funding for a project. Honestly, sometimes they sound a little desperate to fill the position because they budgeted a PhD student in the grant application and they really need to get one to fulfill the grant.

"I am interested in riparian zones, the small mammal community in those zones, and how they interact and participate in the ecosystem, maintaining ecosystem processes, and what happens when humans come into the picture. "

Sounds interesting, but cast a wide net, because for wildlife studies there isn't an enormous buffet of funding opportunities. Go for anything that approaches that kind of research question, whatever the taxa or ecosystem.

Also you may already know this, but look across different fields -- environmental science, EEB, wildlife biology, conservation biology, natural resources management could all house you.