DISCLAIMER: Those who have not had the good fortune of watching Marvel's latest offering, Guardians of the Galaxy Vol. 2 are advised to avoid this question for the major spoilers it contains.
In the movie, it is shown that when Ego first met Meredith Quill in his human form, he looked like a young man, with jet black hair and smooth skin. After Ego is finally able to make contact with Peter 34 years later, he does look like he has aged that much, with grey hair, a grey beard and wrinkled skin. He is a celestial who has been around for millions of years, so why would his human form age like a normal human in those 34 years.
His human form is clearly not bound to the same biological limitations as other humans,
because when Peter shot at him following his confession of being responsible for Meredith's death, Ego temporarily took the form of a much younger looking David Hasselhoff.
So Ego should be able to look as young or as old as he pleases. Why then does he embody the look of an aged man?
Out of universe, this would be explained by Kurt Russell's actual age and appearance. The flashback scenes required a lot of make-up to make him look young which would not be feasible on a regular basis as the bulk of his scenes are in the present day. I am looking for an in-universe answer for Ego's appearance in the film.