According to my parents, it was at the Second Avenue Deli, in Manhattan’s East Village, circa 1977. I was three years old. I had a knish.
I was small enough to be in a highchair and not talking much, so probably a year old or less. My parents liked to tell 2 stories about my early restaurant experiences, which seemed to take place in pancake places, so most likely IHOP and/or Bickfords:
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I like lemons. I have always liked them, and used to freak out the waitstaff by eating lemon slices without frowning.
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Waitresses would always ask my parents if they could give me Saltines while we waited for food, and Dad would tell them that they could if they wanted to but I would make a mess. They never believed him and I would methodically crumble them up and delicately drop the pieces in a circle around the highchair. The waitresses were dismayed.
Years later Dad and I watched Supernatural, and after a plot involving salt lines I amused him by asking him if he and Mom ever considered that I was trying to form a protective circle around the highchair to ward off ghosts 
Both of my grandkids like to suck on lemon slices. I have to wonder if it’s a case of their taste buds not yet being fully developed. At least they’re not licking the salt shakers…
It was the Paramount Steakhouse at 44 Charles Street in Boston’s Back Bay, at the foot of Beacon Hill, 20 miles from home, around 1958, after I got my first (hand me down) car (a '50 Ford). Ate there a couple more times, not back to that restaurant since that time period.
The restaurant is still there according to Google street view.