There is another important possibility; you only just became a programmer and are inherently "late to the party" when it comes to Github. I don't see how anyon

There is another important possibility; you only just became a programmer and are inherently "late to the party" when it comes to Github. I don't see how anyone could use a join date as a measurement of someone's skill as a programmer, it reflects nothing. However in context they may be right. If you're talking about visibility then having only just joined may result in your exposure on the site being smaller due to having had less time to produce work that would attract attention.