Is it okay for a Christian to celebrate their 21st birthday with alcoholic beverages, provided they do so in moderation?
“Moderation” is an important word in your question. The Bible does not forbid the use of alcoholic beverages. It forbids the abuse of alcoholic beverages (Proverbs 23:20, 31-33; Ephesians 5:18).
A 21st birthday—or any birthday—is an opportunity to thank God for another year of grace. We want to express that gratitude with consistent Christian attitudes and behavior.