Can girls play in mixed teams, and up to what age?

Yes. Cricket Australia's Junior Club Cricket stages are open to girls and boys, and Cricket Australia lists junior club cricket for girls aged 8 to 18. Cricket Australia's junior formats do not set a single national age rule for girls playing in boys' or mixed teams: each association or competition sets its own team types and age groups. Examples from Cricket NSW show how this works:

• Mixed club teams: the NSW Junior Community Cup is open to Under 12, Under 13 and Under 15 mixed teams, and separately to Under 13 girls' teams.
• Girls can be a year older at carnivals: in the Cricket NSW Metro Community Carnivals, age is worked out at 31 August and girls' maximum ages are one year above boys': Under 11 (Stage 1) boys 11 and girls 12, Under 13 (Stage 2) boys 13 and girls 14, and Under 15 (Stage 3) boys 15 and girls 16.
• Girls-only representative teams: the NSW Female Youth Championships have an Under 13 girls' shield (Mollie Dive), an Under 14 girls' competition and an Under 15 girls' shield (Margaret Peden), each for girls under that age as at 31 August.
• Girls-only club teams: there are many girls-only junior competitions around NSW, with Cricket NSW describing junior cricket for girls aged 8 to 18.

Once players move to Stage 3 (Under 14 to Under 18), the same match format applies, but female players use the 142 g ball instead of the 156 g male ball.

Sources: play.cricket.com.au For Women and Girls and Junior Club Cricket pages; Cricket NSW Metro Community Carnivals, Junior Community Cup and Youth Championships female playing conditions; cricketnsw.com.au/women-and-girls.