If you work for a company that has a contract with a preferred rental car company, your corporate account *may* have a waiver for employees who are renting using the company contract code. (Not all corporate contracts have an under-25 waiver, but many do.)
You can often use this contract number even if you are not travelling on official business. It depends on what your company's policy is, so even if you don't normally travel for business, ask whoever handles your corporate travel billing.
Note that even with the fee waiver, your company's negotiated rate would normally be a daily rate, not a weekend or weekly rate, so using that rate might end up costing as much (or more) than just paying the under-25 fee with a discounted leisure rate. You should do the math carefully.