The legendary hunter becomes young again is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

If only I were 10 years younger…!! A special reward is given that reflects the target's wish. Gwak Pilseong, a legendary hunter enjoying his retirement at the age of 80. He accidentally gains the ability of immortality and rejuvenates, returning to the hunter industry after a long hiatus… There are… too many rotten scoundrels in the hunter industry!