Hi [Recruiter Name],
I hope this message finds you well. I completely understand if this is not possible, but I would like to request an extension on my offer decision. While there is no doubt that this is a gracious offer and I am excited about the opportunity to potentially join your team, I would like to be sure I am considering all of my options (Note: you can even say you want more time to discuss with a mentor/parent–up to you!) prior to accepting. Please let me know if I can get a decision to you by [Insert Date].
Thanks so much and I look forward to hearing from you!
Best,
[Your Name]