I think because it's a family plan, people don't see it as truly paying for the kid's phone, just keeping them included in the family plan.  27 with a good job seems plenty old enough, but depending on your financial situation, paying for his phone doesn't seem like a hill to die on to me.

SS always had to pay for his own if he wanted one but back when he was a teen we really couldn't afford to have him on our plan.  Back then we couldn't afford the unlimited talk and text plan.  So he got a prepaid phone when he got a job and got his own contract when he could afford it and was old enough. Now that plans are affordable we'll be dealing with this with our bios.  I don't know how we'll handle that yet.  Definitely once they move out on their own they'll need to get their own accounts.  Maybe once they graduate college?  I don't know.  Or if they want a different unlimited plan than what we have we'll tell them they can get their own and pay for it themselves.  

Does this mean D'S should bump my husband off his plan?  Lol.  We have the opposite situation here, SS works for a phone co.

When they have a job. 

When they have a job and are earning, they should pay for their own phone and plan.

Stop the coddling...

That being said, when SO wants to pay for adult SD24 Bratty McBratFace's cell phone bill - then great, no family plan, adult kid is on separate plan paid for from his own money, not joint funds.

Same with Netflix.  Oh, SO, you want to keep your adult child freeloading that?  No problem.  I'll freeload it too.  That gets paid from his money, not joint money.  And I get to freeload too.  See, it's important that blended families have some degree of separation of funds.  Then these 'issues' become easy to solve.

All three of ours started paying 3 months after turning 16; the age to get a job and take on more responsibility. Once they move out (SD19 already did) the have one month to remove their phones from our plan or the line will be cancelled.