Tsunade uses her reserve seal to keep herself young but its effect wore off when she had to spend all her chakra on healing herself in the war. Here's the catch, for every injury Tsunade heals with her Creation Rebirth Technique, her lifespan shortens. Meaning her body is older than the time she has been alive.
Tsunade is actually really old, but the thing on her forehead (Ninja Art Creation Rebirth — Strength of a Hundred Technique ) allows her to look younger than she actually is. So during her fights, if she uses up all her chakra it causes her to look old. Also every time she used the technique, she looked older.
Naruto Profile: TsunadeeditTsunadePersonalBirthdateAugust 2SexFemaleAgePart I: 51 Part II: 54–55
p./pp. P. is a familiar abbreviation for page and so is its plural pp. (meaning "pages," as well as "per person").
(also p.p.) Per procurationem (used when signing a letter on someone else's behalf).