Thanks for your query.
There is no reason for swelling in legs at this age.
If there is no history of any injury i suppose you should get a consultation from a general physician
and get her checkup done.
It is also important to rule out thyroid problems as you say she is slightly overweight and there is swelling.
Do not hesitate to contact me if you need any further assistance.
Thanks & Regards
Dr Akshay Kumar Saxena