A twelve-year-old girl is asking about the ruling on doing sports in mixed venues

How good it is that this Muslim girl, who is so young, is
keen to ask about matters of her religion and is keen to make her life,
behaviour and attitude in accordance with her religion. Whatever her
religion allows, she does it and avails herself of this concession.
Whatever Islam does not allow, it is best to keep away from, and what
Allah has permitted His slaves is sufficient and there is no need to
transgress His limits and do what He has forbidden. 

It is clear that one of the aims of sharee‘ah is that women
should cover themselves and guard their chastity, and that a woman should
not show her charms to a man who is a stranger (non-mahram) to her. 

It is well known that when girls do sports, especially games
involving strength or some other games which need particular effort, when
they do these sports in mixed clubs and venues, it leads to the girl showing
a great deal of her charms, and showing that which is not appropriate,
especially when these exercises cannot be done wearing full jilbab. 

We have discussed the guidelines on women participating in
sports, the most important of which is that it should be out of the sight of
non-mahram men. See the answer to question no.
115676. 

Doing the exercises that a woman needs at home means that she
has no need to go to clubs and open places, because that usually involves
uncovering and mixing. 

In the answer to question no.
1200 we have discussed the
prohibition on mixing because of the serious bad consequences to which it
leads. 

It should be noted that many girls reach puberty and thus
become fully accountable when they are twelve years old. If she has not yet
reached this stage, she is very close to puberty and has reached an age at
which men may be attracted to her. A girl like this should cover and avoid
mixing with men and so on, just as is required of an adult woman. 

See the answer to question no.
43485. 

The Muslim woman should be keen to do sports that are in
accordance with her nature and femininity and avoid violent sports which may
affect her body and may cause her real harm. 

We ask Allah to guide you to that which is good for you in
this world and in the Hereafter. 

And Allah knows best.