How old was melchizedek from The Bible?

Ageless or Eternal. Paul makes an analogy to Melchizedek being Jesus as the King of Righteousness - and there is only one:

Hebrews 7:1-3New King James Version (NKJV) Hebrews 7

The King of Righteousness

1 For this Melchizedek, king of Salem, priest of the Most High God, who met Abraham returning from the slaughter of the kings and blessed him, 2 to whom also Abraham gave a tenth part of all, first being translated "king of righteousness," and then also king of Salem, meaning "king of peace," 3 without father, without mother, without genealogy, having neither beginning of days nor end of life, but made like the Son of God, remains a priest continually.