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With federal student loans, there is no limit to how long they can collect the loans. These individual negative marks have aged off your report, but the loans could be transferred to a new collection agency and that CA could start the reporting process all over again. Since that's the case, the best thing to do is act quickly while they aren't reported and get them out of default so they can't be reported again.
If you aren't in a hurry to buy a house, I would still rehab. Rehab removes the most penalty interest/fees, and it also results in backdated tradeline that can improve your credit score by boosting your average age of accounts. Once you've started rehab, there's really no reason for the CA to change or report again, so you shouldn't have any additional negative information on your report either.