The sooner the better. My first internship was the summer I turned 18, prior to me starting uni. It absolutely set me apart and allowed me to pick and choose opportunities moving forward.
When sending resumes, I'd say it depends on the companies you are applying to. If you can seek out direct lines of communication with people involved in the hiring process (i.e. career service dept) then do so without delay.
If you are applying cold then do your best to find out when they start hiring their interns and apply at the beginning of that window. If you can't find out, then apply immediately and follow up routinely.
Show some initiative. Be the squeaky wheel. You will be better off.
