Where was Renly Baratheon during Robert's rebellion?

Question

Where was Renly Baratheon during his brother Robert's rebellion? I know Stannis, the middle brother, held the castle Storm's End, through a long seige.

Answer 1

Storm's End

He was there during the siege and he recounts the following events that took place during that time. Note that they are talking about cannibalism which would only have happened had they been under siege with no hope of supplies getting through, unless you're a certain smuggler that is.

"Yields?" Lord Rowan laughed. "When Mace Tyrell laid siege to Storm's End, Stannis ate rats rather than open his gates."
"Well I remember." Renly lifted his chin to allow Brienne to fasten his gorget in place. "Near the end, Ser Gawen Wylde and three of his knights tried to steal out a postern gate to surrender. Stannis caught them and ordered them flung from the walls with catapults. I can still see Gawen's face as they strapped him down. He had been our master-at-arms."
Lord Rowan appeared puzzled. "No men were hurled from the walls. I would surely remember that."
"Maester Cressen told Stannis that we might be forced to eat our dead, and there was no gain in flinging away good meat." Renly pushed back his hair. Brienne bound it with a velvet tie and pulled a padded cap down over his ears, to cushion the weight of his helm. "Thanks to the Onion Knight we were never reduced to dining on corpses, but it was a close thing. Too close for Ser Gawen, who died in his cell."
_{A Clash of Kings, Catelyn IV}

Answer 2

At the time of Robert's Rebellion, Renly was still very young. He was born in 278 and the rebellion happened in 282, so he would have been 4 through 6 years old during the war.

A child that young would have made a perfect hostage, so he was presumably kept under lock and key in Storm's End the whole time.

By comparison, Robert was born 263, and began the rebellion at age 19. Ned was the same age, though he was the second-eldest; Brandon was 21 when he was killed.

Stannis was born 265, and thus was 17, plenty old enough for the Westeros military.