If you're over 59 1/2 years old, you can get as much money out of your 403(b) plan as you want, including to make a mortgage down payment. See, when you hit that magical age of 59 1/2, you can take qualified distributions from your 403(b) plan, so you won't owe any early withdrawal penalties. But, you will still owe income taxes on the distribution.
You could take a hardship distribution. But that only applies if you have no other way to pay for the down payment. But consider, if you could take out a loan, you wouldn't be able to take a hardship distribution. But, if a loan would disqualify you from your mortgage, you can take a hardship distribution.
There's no exception for distributions taken from your 403(b) plan for a mortgage, even if it's your primary residence or even your first home. So, you will owe income taxes on the hardship withdrawal/distribution.
Your plan might offer you the opportunity to take out a loan from your plan. You can borrow up to $50,000 or half your vested account balance, whichever is less. Typically, loans require repayment over five years, but when you use the proceeds for your down payment on your main home, you can take longer. Plus, the interest you pay goes back into your 403(b) account. The downside is that if you fail to repay the loan or default, it counts as a permanent distribution.
See IRS for loan limitations and repayment requirements your administrator and you should be aware of: https://www.irs.gov/retirement-plans/403b-plan-fix-it-guide-you-havent-limited-loan-amounts-and-enfo...
