Two points here.
First, are we saying that Shivshakti is too young to be playing the part? If so then perhaps that quite true. They could have got someone a little older to play the role of Bebo.
Secondly, are we saying that the character herself is too young to be dealing with track such as marriage and romance? Bebo is supposed to be 18 right? At 18 you can't be considered a child, and as for being too young for marriage and romance ....it would depend on the individual. So in terms of the character herself, I think it's neither negative nor positive, but merely a fact of life that some girls do get married young.
Having said that Shivshakti does have too much of a baby face to be playing the part of an 18 year old about to get married. But here again, I know plenty of girls who look like they're about 16, but are infact in their early twenties (must be nice to be blessed with such good genes!)
From the creatives part, I think they wanted a girl as cute and young as innocent looking as possible to highlight that she really is everyone's laadli. Visual image is everything. As a viewer I'm more likely to believe that Bebo (played by Shivshakti) is everyones laadli, more than if she were to be played by lets say the girl who plays Prabjoks part.
keeping this in mind I tend to dissassociate the two when watching. Reminding myself that the character herself is 18, even if the actress playing the part looks like she just left kindergarten!
Not saying it's right or wrong either way, merely offering my view on the matter.
And a list on things that cheese me off about desi telly would be a list too long. Let's not even get started on the whole 'women are either pure as snow, or black as sin'!