24/7 Wall St.'s article explains why a hypothetical 61-year-old hoping to see Anthropic in his S&P 500 401(k) right after its IPO may wait much longer: S&P rules require at least 12 months on an eligible exchange plus positive GAAP net income, and the index committee rejected shortening the wait for megacaps, so the fund could take years to buy a share. 24/7 Wall St. says trading begins November 19, 2026, and advises checking whether a 401(k) offers a Nasdaq-100 or Russell 1000 fund and capping any direct purchase at a loss one can absorb.