Is my favourite poem rudyard Kipling’s if Still relevant today?

Yes, because it tells the basic rules of life.  In fact, the older that I get, the more of the poem that I understand.  

I think if the male content was was replaced with something more gender neutral it could fit in today’s society.

Then you will an adult my child lol

Many ppl like it

The underlying message still has relevance even if its highly patriarchal overtones have not. To give it relevance, one must work hard to turn a blind eye to the misogyny that pervades it, to the the categorical discounting of women so as to not even give women the slightest consideration in such lofty aspirations and accomplishments, for a woman could certainly never have the Earth and everything in it, only a son becoming a man.