About how old was Jairus's only daughter when she was dying?

Answer: Twelve

Jairus's only daughter was about twelve years old and dying when he came to Jesus.

“Just then a synagogue leader named Jairus came and fell at Jesus’ feet. He begged Him to come to his house, because his only daughter, who was about twelve, was dying. As Jesus went with him, the crowds pressed around Him,”— Luke 8:41-42 (BSB)