Losers in election contests have standing to challenge the outcome of the contest, if the challenge is based on fraud or eligibility. That’s universal, as far as I know. As of right now, Trump has not lost any election contest, caucuses being party affairs, and not state election affairs.
Even if Trump loses the primary then how does he demonstrate the damage is due to Cruz being on the ballot as opposed to Kasich, Bush, Carson, or Rubio?
Could a political party deny the constitutional rights of someone vying for the nomination within that party by allowing a candidate, who is ineligible, to run?
It is possible that this may only be litigated by someone running in the general.
Laurence Tribe might be right; it may not be justiciable.
I sometimes wonder what if the primary candidate were 32 years old rather than 35? Same answer?