My friends had a baby last year, and he's turning 8 months by April 15, 2016. The parents are going to file a joint tax return this year. Can they claim their baby as a dependent? Should they consider filing separately? If they claiming him as a dependent, do they need to file a tax return for him too?
As long as the baby was born by December 31, 2015, he can be claimed as a dependent by his parents for tax year 2015. He should definitely be claimed as a dependent; it will save them money on their taxes, and there is no reason not to.
The baby does not need to file a tax return, but he does need a Social Security number in order for the parents to claim him as a dependent and claim child tax credits. Often Social Security numbers are applied for at the hospital right at birth, so he might already have one, but if for some reason this was not done, they will need to visit a Social Security office to apply for one.
The question about whether or not a married couple should file jointly has nothing to do with the fact that they had a baby. If they are planning on filing jointly, we don't have any information that would suggest this is a bad idea.
Whether someone needs to file a tax return for a given year is covered in a section at the beginning of each year's Form 1040 instructions, called "Do You Have to File?". Generally, if the income that year is below a certain threshold (this threshold varies depending on filing status, whether the person is claimed as a dependent, and other factors), then the person isn't required to file (though they could still choose to file if they want to).
For a single dependent under 65, they must file if they have $1050 of unearned income, or $6300 of earned income, or their gross income is more than $1050 and more than earned income + $350. If your friend's baby didn't have income for 2015, then the baby won't need to file.
Whether a person needs to file is separate from the consideration of whether the person can be claimed by someone else as a dependent. Some children who are claimed as a dependent nevertheless must file if their income is high enough.