Age-. The general rule in the industry is that once an A/C unit reaches 10-15 years, the idea of replacing the unit is usually a much better investment than putting money into an older system. If your system is at this threshold, it's usually wise to begin expecting, not wondering, when the system will fail.
You can keep the money in the TSP after you leave your federal job to benefit from the low fees, or you can roll over the money into an IRA or a new employer's 401(k), 403(b) or 457.
There is an option to roll over the age-based withdrawal to an IRA or other qualified plan. If you wish to have greater choice in investments than the TSP offers, this is one way to accomplish it. You might also simply want to begin consolidating your retirement investments late in your career.