[masaegu]

Both are correct. It's the classic 大和言葉 vs.　漢語 issue.

白髪のおっさん　＝　しらが

白髪の紳士　＝　はくはつ

"Darn, I'm getting grey hair!" ＝　しらが

Frequency-wise, one would hear しらが considerably more often.