You can try converting to a byte (range 0 - 255) and handle the error from out of range values... unless you plan to allow an age of more than 255 years.

Code:
Option Explicit

Private Sub Form_Load()
   Do While Not IsValidAge(InputBox("Enter person's age"))
      MsgBox "Invalid value for age"
   Loop
End Sub

Public Function IsValidAge(strAge As String) As Boolean
Dim bytAge As Byte

On Error GoTo ErrHandler
   bytAge = CByte(strAge)
   IsValidAge = True
   Exit Function

ErrHandler:
   Err.Clear
End Function