Unless you change your family situation, you will file as "Single" next year and for years moving forward. Other filing status will not apply to you. However, you should revisit this conclusion if your family situation changes such that you pick up a future dependent who can be classified as a qualifying child or a qualifying relative. Here is an IRS link that can assist you for filing status: https://www.irs.gov/help/ita/what-is-my-filing-status
Yes, the income limit changes when Single. This year, the Single, over age 65, income number for filing is $13,600. This is basically just the Standard Deduction number. At that income point, your Social Security income will not be taxable and will thus not count toward the income number. Here is the link to the IRS that can provide additional assistance: https://www.irs.gov/help/ita/do-i-need-to-file-a-tax-return
Here is a more complete treatment of filing requirements. However, your description of your situation does not appear to include these cases.
You must file a return if any of the conditions below apply for 2018.
1. You owe any special taxes, including any of the following.
a. Alternative minimum tax.
b. Additional tax on a qualified plan, including an individual retirement arrangement (IRA),
or other tax-favored account. But if you are filing a return only because you owe this tax, you
can file Form 5329 by itself.
c. Household employment taxes. But if you are filing a return only because you owe this tax,
you can file Schedule H by itself.
d. Social security and Medicare tax on tips you did not report to your employer or on wages
you received from an employer who did not withhold these taxes.
e. Recapture of first-time homebuyer credit. See the instructions for Schedule 4 line 60b.
f. Write-in taxes, including uncollected social security and Medicare or RRTA tax on tips
you reported to your employer or on group-term life insurance and additional taxes on health
savings accounts. See the instructions for Schedule 4, line 62.
g. Recapture taxes. See the instructions for Form 1040 line 11 and Schedule 4 line 62.
2. You (or your spouse, if filing jointly) received HSA, Archer MSA or Medicare Advantage
MSA distributions.
3. You had net earnings from self-employment of at least $400.
4. You had wages of $108.28 or more from a church or qualified church-controlled organization
that is exempt from employer social security and Medicare taxes.
5. Advance payments of the premium tax credit were made for you, your spouse, or a dependent
who enrolled in coverage through the Marketplace. You or whoever enrolled you should have
received Form(s) 1095-A showing the amount of the advance payments.
