What do you think the ages have to do with it?
Only a legally married couple can file a joint return.
If you were legally married at the end of 2018 your filing choices are married filing jointly or married filing separately.
Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,000 (+$1300 for each spouse 65 or older) You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI) If you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.
are you legally married?
The IRS recognizes common-law marriages as legal marriages. A common-law marriage exists if you and your partner live together as husband and wife, but there's a fine line between a common-law marriage and just living together. A common-law marriage involves a mutual agreement that you're married, as well as holding yourself out to society as a husband and wife.
Fifteen states and the District of Columbia recognize these marriages, including Alabama (if created before January 1, 2017), Colorado, Georgia (if created before January 1, 1997), Idaho (if created before January 1, 1996), Iowa, Kansas, Montana, New Hampshire (for inheritance purposes only), Ohio (if created before October 10, 1991), Oklahoma, Pennsylvania (if created before January 1, 2005), Rhode Island, South Carolina, Texas and Utah. If you have a valid common-law marriage, you are considered married for tax purposes.
And each of those states have their own requirements as to what constitutes a valid common law marriage. If that is what you think you are then I suggest getting the advice from a family law attorney in your state first. In addition, such a marriage, if valid, is the same as any other marriage and can only be ended by divorce.