[RPG] How to manage the Druid’s wildshapes

Timeless Body is what Wizards of the Coast has called a "Ribbon", i.e. a neat ability with mostly story, rather than mechanical, applications. Another ribbon ability is the rogue's Thieves' Cant, though that has more obvious benefits to the campaign at large.

See the sidebar in the Waterborne Adventures UA article here for a full explanation.

I can't think of many things that interact mechanically with timeless body. It wouldn't be unreasonable to rule that it also reduces aging from various mechanical effects, like Wild Magic Surges or potions of longevity, but aside from those edge cases, which are arguably subject to ruling anyway, the ability's mostly for fluff.

In story, I can see it causing very old archdruids running around the setting.

Masters of the Wild web enhancement - Druids Grove (3.5e), which was a pdf addon to the corebook Masters of the Wild, contains the following info:

Must haves:

• First, there must be a place for the druid to sleep. This can be a natural shelter, such as a cave, or one built from natural materials, such as a sod hut or tree house.
• Next, there must be a good supply of food and clean, fresh water. The food supply can include nuts and berries, tree fruit, and a diverse population of animals.
• In addition, most druids maintain at least one herb garden, and many also cultivate a patch of vegetables.
• The next vital feature is a system of protection for the druid and her charges, both plant and animal. This often takes the form of natural barriers, such as briar hedges or dense forest.
• Warmth is another vital issue. If the climate where the druid lives is temperate or cooler, she requires a good source of firewood, coal, or other fuel. In addition, druids use natural materials to make warm clothing and blankets. Each generally has a cache of tools stashed away for sewing hides and weaving cloth from plant fibers or animal hair.
• Druids like to be prepared for emergencies, so most keep a supply of important magical and nonmagical items secreted about their groves—in hollow trees, under stones, and in unused animal lairs. Stashed magical items usually include goodberries and infusions of useful spells (see Masters of the Wild, Chapter 3). Other useful items include small amounts of money (in case they need to visit a town), tools for starting a fire, knife and extra weapons.
• A typical druidic grove features some means for scrying—most often a pool of still, mirrorlike water. A large, highly polished rock also might make a good scrying device. In a pinch, a druid can use the create water spell to produce still water for scrying in a large container or natural crevice.

Those are the main points, slightly edited for brevity. The rest of the pdf is devoted to outlining a sample druid's grove for a 12th level Half Elf Druid named Thyme. The pdf is freely available for download at the link above.

In addition, if you have access to the 2e rulebook The Complete Druid's Handbook, there are details on lesser and greater magical groves, which would add the benefits you are seeking, among which are the following (Lesser Grove):

• Druids entering the grove feel a watching presence and a sense of power. For every three rounds they spend within its boundaries they learn one power of the sacred grove, through a vision or intuition.
• All druids receive a +1 bonus to saving throws vs. spell, death magic, and wands while within a lesser magical grove. The grove’s steward receives a +2 bonus. All in the grove are rendered immune to magical fear while within it.
• Dig spells never work within a grove.
• Natural (nonmagical) lightning never strikes trees or beings in the grove.
• Evil enchanted creatures cannot enter the grove unless it has been defiled. (See ”Defiled and Cursed Groves” in this chapter).

There are tables to determine random benefits for lesser and greater magical groves (Of which not all groves are magical to begin with.)