There is no argument against it for people who naturally retain one and are not balding. I saw a 72 year old black guy with a juvenile hairline and it looked fine. There are some relatives on my mum's side who have it too. It looks good. I had the option of attaining one and my circumstances being ideal (donor numbers, hair final pattern, finasteride, hair diameter, age e.t.c).
But these are things that led me not to opt for a juvenile one:
1) Once balding, you may get some further balding and a juvenile hairline with thinning in other areas, is a bit of a give away. Of course, corrected with further surgeries though, but it's a waiting game and things may become apparent on social media.
2) You save grafts which you can potentially use later and money.
3) Mature hairline generally looks more natural especially once you have male pattern baldness. Will ensure the hair transplant is more subtle. People will see an improvement but not necessarily know where it came from.
4) I saw pictures of me with juvenile and mature and concluded I preferred the mature one to complement angulated facial features. I could pull off both though.
5) Surgeon drew both (it's important) and also felt the mature one was better. He said though it must be Norwood 2 or below to be non balding and appropriate grafts per cm2 - can't emphasise how important this is - don't walk away without this. Dr Rassman and the Norwood status chart also say balding begins after Norwood 2 so I was comfortable with a 1.5 as I had done my research.
Side note: mature is down to measurements and not guess work. They make you wrinkle your forehead and draw the juve one. After that, they place a mature one about half inch higher in the middle and maximum 1 inch higher in the corners. Or some measure in a different way: rule of 1/3s and eye brow distance. You get what you pay for.
edit: and forgot to add, get some temple points at least, especially if they are completely gone.