'expat the timing of baby Aegon's birth relative to Dany's is irrelevant, it's about being the son of the firstborn son of the king rather than being a subsequent child of the king. Prince Andrew is older than Prince William but William is next in line to the throne after Charles.'
I was referring to something else and two males, but not wanting to give out too many spoilers to those who hadn't read the books. I'm perfectly aware of how primogeniture works, thanks, although as pointed out it does not apply in Dorne, where Ariane is heiress rather than Trystane.
Aegeon is older than Dany, of course, but he is the son of who he is and that's what counts. No telling about Jon, it's entirely possible he is Rhaegar's and Lyanna's and that she was second wife to him in Targaryen fashion, hence the whole Tower of Joy, 'Promise me, Ned!', her bed of blood and that she was heard screaming as Ned and his 6 men approached Ser Dayne and his two companions.
A possible problem might be that people don't believe it is really Aegeon, especially because Varys was the one who orchestrated the escape and many don't trust him, despite the earnestness of Connington, who may be dead soon enough, or the other witnesses.
If you believe the Maegi then all Cersei's offspring are toast even if the latter doesn't succeed in killing off Robert's remaining bastards (Ned unwittingly gives away the most current whereabouts of nearly all of them to her in the first book, resulting in the death of baby Barra) in time before her badly kept secret is revealed.