Standard Deduction: $13,500 for 65 and older, if my sources are correct. $12,200 plus $1300 for 65+.If your income does not come up to the standard deduction ($12000 for single), you need not file. I would assume you know what your gain/loss of sale of stocks was.
However, the 'warning' is that if you don't file, you are more open to someone filing in your name and not knowing it, meaning opening yourself up to potential fraud.
BTW, I posted kind of tongue-in-cheek and failed to mention that wife is an AARP tax aide (and the warning about filing regardless of low income is given to all clients). She was also an accountant in a former life.