There might be some sort of 'official' cut-off date, but personally I have always thought of pre-World War One, and certainly the Victorian period as 'early', and post World War One through to the 1960's as 'vintage'.  This ties in with the European definition of  'antiques' generally, which are only antiques if they are 100 years old or more.

I'd agree pre WWI is a good reference, but then a 1920's underwood is virtually the same machine as it's earlier pre war counterpart  I think though despite contrary opinion, it might be better to simply set a date such as 1914 as a definition for "Early typewriter" I'd also venture that the Underwood No.5 considered the father of all modern typewriter might be considered the benchmark as it foreshadowed and defined the modern typewriter from which all subsequent models derived. I consider an antique as being one hundred years or older whereas vintage is just out of style...