First, Dumbledore wanted to have time to talk to Harry about his scar.
“So tell me, Harry,” said Dumbledore. “Your scar . . . has it
been hurting at all?”
Harry raised a hand unconsciously to his forehead and
rubbed the lightning-shaped mark.
“No,” he said, “and I’ve been wondering about that. I
thought it would be burning all the time now ...
The “five years” isn’t counted from when they last met.
However, there can be no doubt that Dumbledore delayed, for some five years of turmoil, fatalities, and disappearances, his attack upon Grindelwald.
This line refers to a period from about 1940 to 1945. Grindelwald caused turmoil, fatalities, and disappearances during this time. Dumbledore hesitated ...
In Chapter Thirty-Five of Deathly Hallows Dumbledore explains the timeline more clearly:
“The argument became a fight. Grindelwald lost control. That which I had always sensed in him, though I pretended not to, now sprang into terrible being. And Ariana... after all my mother’s care and caution... lay dead upon the floor.”
“Well, Grindelwald fled, as ...
When Dumbledore and Grindlewald first met they were both young teenagers. When they both parted after the incident around Ariana Grindlewald was not yet the mighty dark wizard he was about to become much later.
He already had his ideas about the „greater good“, but he did not have his power yet. Later -about 40 years later- he became the (in)famous dark ...
They dueled (at least twice).
The first duel in which Arianna was killed happened when they were young and ended their friendship. Afterwards Grindlewald went on to wage his war, which was ended when he again dueled Dumbledore in 1945.