If you go to Reporting > Custom Report > New Report, on left side under General Filters, select the year group(s) you wish to report on (or leave blank for all years).
On the left side, on the Dates tab under Filters, tick "Summer Born" and then Run Report at the base of the page.
An alternative method is to go to Reporting > Reports > Students (tab) > Birthdays and enter the summer dates into the date range filters (e.g. May till August)
This will show you all the students with summer birthdays between those dates. If you click on the Year header (avoiding the dustbin icon), this will filter the students into year groups.