I just Googled Hilton Check In Age and the following came up:
Hilton Chicago - Pet Friendly Michigan Ave Hotel Policies
Minimum Age to Register, 18. Check-Out Time, 11:00 am. Early Departure Fee, $0.00 One night room + tax unless otherwise contracted. Late Check-Out Fee …
Hotel Policies - Hilton Garden Inn
Check-In Time, 3:00 pm. Minimum Age to Register, 18. Check-Out Time, 12:00 pm. Late Check-Out Fee, $50.00 Based on availability …
Palmer House a Hilton Hotel | Hotel Policies
Check-In Time, 3:00 pm. Minimum Age to Register, 18. Check-Out Time, 11:00 am. Early Departure Fee, $0.00 Contact hotel. Late Check-Out Fee, $0.00 Contact …
San Francisco Union Square Hilton Hotel Policies
Minimum Age to Register, 14. Check-Out Time, 12:00 pm. Early Departure Fee, $100.00 Applies to departure date changes made after check-in. Late Check-Out …
Hilton Chicago/Oak Brook Suites | Hotel Policies
Minimum Age to Register, 18. Check-Out Time, 12:00 pm. Early Departure Fee, $50.00 Any change to departure date after check-in is subject to an EDF.
It is obviously NOT a Hilton-wide policy that check in age is 21; it appears to be a hotel by hotel policy decision. I have seen Hilton properties with a 21 check-in age, but it is definitely not universal. I was a little surprised to see a 14 check in age in San Francisco… that might be a typo.