See Allowed Variable names in the manual: identifiers cannot start with a number (including numeric subscripts).
Every programming language has some restrictions on the characters allowed in identifiers, and Julia is more permissive than most. (Python doesn’t allow numeric subscripts at all, e.g. x₃ is not a valid Python 3 identifier. Similarly for most other languages, thanks to a fairly restrictive recommendation from the Unicode consortium.)
See this issue for discussion on allowing identifiers to start with numeric subscripts/superscripts.