What kind of company has a 401K that you can’t enroll in for more than 6 months? Let alone 18 months. And what are the details around the plan?
If you started this summer, I could maybe see missing some arbitrary 6 months cutoff. Even then, most companies will let you enroll and start contributing to the 401K, you’re just not eligible for match until the 6 month mark.
I’d say take a good long look at the plan, verify the management of it, match, vesting, expense ratios, etc. and most importantly, getting your money out when you leave.