We are active 70 yr. olds who love the outdoors and wildlife. We're planning a trip to Thailand in Jan. 2027 and looking for a travel agency to work with in Thailand for setting up guide/drivers, hikes in national parks, and the best use of our time, maybe 10 - 12 days. We'd like a private tour. Thank you for recommendations based on your experience.

Can't help with tour companies, but regarding destinations... National Parks...
Most tours will recommend Khao Yai (northeast of Bangkok) and Khao Sok (south, Surat Thani) – these places are fine but now VERY popular.

Thailand has hundreds of national parks; most are under-visited by foreigners.
I'd check out the Western Forest Complex (WFC) – this extends from northern Chiang Mai, Mae Hong Son, right down the Burmese border to near Hua Hin.
Some parks to look at are Mae Wong (Kamphaeng Phet/Nakhon Sawan), Thi Lor Su Waterfall (Umphang, Tak), Kaeng Krachan (Phetchaburi and Prachuap Khiri Khan) and Sai Yok (Kanchanaburi) . Kui Buri in the South (Prachuap Khiri Khan) has wild elephant herds
Look at Google Maps to find these places and then see what accommodation is nearby.

January is the "cool, dry" season, though it might be a little wet on the Gulf of Siam coast in the south.

A friend said this lot was good – Exodus and also EXplore. Google them; they seem to cater to older travellers and have an awareness of local culture – always be aware of animal exploitation in Thailand especially elephant abuse.

Thank you. Information is always appreciated.