Why did Ego's human form age normally?

Question

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.

SPOILERS AHEAD

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.

Answer 1

Since we don't have access to scripts or anything, we'll just have to go at this unsourced, but it will make sense.

What are Ego's powers when it comes to forms?

We see clearly that Ego can change his form as he wills. He swaps from Kurt Russel to David Hasselhoff in a heartbeat. He also would've taken other forms to mate with all the other species as he would've aimed at appearing attractive towards them to mate.

This is confirmed by James Gunn in an interview with Complex where he says

Ego is a living planet who is able to manifest himself in different ways he’s been alone for millions of years and he’s learned how to control the molecules around him in such a way as to create avatars of themselves, such as Kurt Russell. _{James Gunn interview with complex}

So why young Kurt Russel?

Ego would've chosen to look young and "cool" to ensure he mated with a human of his choice. Due to his abilities, he would've been able to adapt himself to be attractive to any human of his choice. For his "River Lily" he seems to have chosen a young Kurt Russel, which worked.

That makes sense, but why did he age?

Ego likely would've thought that had he aged in the same way a Human would, he'd be more approachable to Peter, more like the father figure Peter was missing. He wanted Peter to like him so he could use his power. He needed Peter to take over the worlds he planted his seedlings on. This meant that he would need to look like a 34 year older version of himself to make it more convincing to Peter that it was indeed him.

Answer 2

He did it so that Peter and the others could relate to him. Don't forget that when he first appears, it isn't immediately apparent that he is a Celestial, or any being whose life spans longer than that of Peter.

During his errands, Peter has met many alien races, most of which have a life span "similar" to that of humans: for all he knew (even knowing that his father could be of any alien race), his father could have lived maybe twice or thrice as long as him, but he would have surely aged by then, looking somehow older than him.

So, not only did Ego choose to show an aged form of his human avatar, but he deliberately chose to have the avatar age just as much as a human would have, so that it is even easier for Peter (and for his friends) to accept the revelation.

When Ego later reveals his true celestial nature, it becomes evident that his avatar is a deliberate creation of his, yet he keeps a form that looks fatherly to Peter so that he can earn his trust, which he needs to complete his grand plan.

Answer 3

In addition to @Edlothiad's answer I believe when Ego changes from Kurt Russel to David Hasselhoff he says something like:

I simply picked the form that would be most pleasing to you.

Meaning he realised that it wouldn't make sense to Peter if he was still a young man or whatever other form he chose. He had to look like he was his father, meaning he had to look aged otherwise Peter wouldn't believe it was him.

Answer 4

Peter already had trouble believing that Ego was his father when they first met - Gamora had to persuade Peter to go with him.

If he didn't even look like he could be Peter's father (i.e. if he looked the same age or younger), it probably would have been completely beyond belief for Peter, so he would likely have never gone with Ego at all.

Answer 5

Youth is overrated when you are an immortal planet that just wants a kid (so you can conquer the universe).

Why does Ego need a form that doesn't age? Who cares if it ages when it is trivial to replace? Ego can replace and manipulate his Kurt Russell form effortlessly, recreating it after it has been destroyed, and even making it look and sound like David Hasselhoff. He could create an unaging form, or even a form that can leap tall buildings in a single bound. But why?

Ego wants to expand throughout the universe. He cannot expand any further by himself. For some reason he needs another being like himself that is not himself to help him. So another avatar would not work because it is still himself. He needs to mate with another life form to produce offspring that inherit his powers.

This is actually really hard. Ego is a superintelligent, immortal planet that can manipulate matter on a planetary scale, yet this is the hardest problem that Ego has ever faced. He has tried and failed at least thousands of times.

Solving such a problem required Ego to study and understand the other life forms in the universe. He needed a form that is subject to all the biological limitations of the life forms he is consorting with. He needed a form that can live, age, and fall in love like other life forms do. Creating a super-powered or unaging form is not just unnecessary, it is counterproductive. He is far more likely to succeed in his goal with a form that is subject to all or most the biological limitations of other life than with a form that is subject to none of them.

Unfortunately the movie is still only available in theaters and there are no video releases or scripts available so I don't have any sources to back this up yet.