What a healthy weight for 21 year old girl?
It would depend more on your height than age. There are online calculators that can give you a range of "appropriate" weights for your height.
But unless someone assessing your health knows your overall health, body build, per cent body fat vs. muscle, etc , a "healthy" weight is what YOU are healthiest at. Weight ranges are only tools to give health professional a starting point re: your weight, to include in their overall evaluation.
Do some googling for "healthy weight range calculators", but don't obsess about it if you're outside the range. Talk to your doctor if you aren't sure.