SELECT avg(age) ,  min(age) FROM artist WHERE country  =  'United States'
What is the average and minimum age of all artists from United States.
[Schema (values) (types)]: | theme_gallery |  artist : artist_id (text) , name (number) , country (text) , year_join (text) , age (number) | exhibition : exhibition_id (text) , year (number) , theme (text) , artist_id (text) , ticket_price (number) | exhibition_record : exhibition_id (text) , date (number) , attendance (text);