There are 2 parts to this –
- 19 year olds are immature in general.
This is a basic biological fact.
However, like you, I believe there are exceptions to this.
Unlike you(?), I don't believe Diana was an exception to this (as said earlier, fits, tantrums, throwing herself down the stairs, emotional reactions like abruptly cutting off her DS' nannies due to jealousy, confiding in William that being pregnant with William made her want to kill herself, and the list goes on)
- PP's assertion that "many 19 year old women have more sense and maturity than a man in his 30s".
Scientifically it's true that female brain development outpaces male development, but not by over a decade, sorry.
In terms of this specific case, I don't know if Diana was more mature than Charles at the point of marriage, but I highly doubt so (and that's taking into account that Charles was emotionally stunted and dysfunctional as well).