Minimum rental ages usually sit between 18 and 25 depending on the country and company, with a common pattern being rentals allowed from 21 but a daily young-driver surcharge until 25, and premium vehicle categories restricted to older drivers. A license held for at least one or two years is another frequent requirement, so a fresh license can block a rental even for older drivers.
On insurance, the near-universal baseline is third-party liability, mandatory almost everywhere and normally included, which covers damage you do to others. Collision damage waiver and theft protection are usually included with a significant excess, meaning you still pay the first chunk of any damage. The counter upsell is excess reduction to zero, which is convenient but pricey per day; standalone excess insurance bought in advance typically does the same job for less. Some credit cards include rental car coverage if you decline the rental company's CDW, but the conditions are strict, so verify yours actually applies in that country before relying on it.
Whatever you choose, get the coverage summary in writing, note the excess amount, and video the car at pickup. Gravel roads, tires, glass, and undercarriage are often excluded, which matters if you are heading anywhere rural.