Recent data on international migration of skilled workers define skilled migrants by education level without distinguishing whether they acquired their education in the home or the host country. This article uses immigrants' age of entry as a proxy for where they acquired their education. Data on age of entry are available from a subset of receiving countries that together represent 77 percent of total skilled immigration to countries of the OECD (Organization for Economic Co-operation and Development). Using these data and a simple gravity model to estimate the age-of-entry structure of the remaining 23 percent, alternative brain drain measures are proposed that exclude immigrants who arrived before ages 12, 18, and 22.