VBA to replace multiple values with specific labels

I'd suggest first creating a 'lookup' table, which maps ages to your required groups:

	A	B
1	Age	Age Group
2	24	20-24
3	25	25-29
4	26	25-29
5	27	25-29
6	28	25-29
7	29	25-29
8	30	30-34
9	31	30-34
10	etc

Then you can use a bit of code to replace age values with age group values:

Sub foo()
    Dim c As Range, wsLookup As Worksheet, rngLookup As Range
    
    'change these values to match your lookup table
    Set wsLookup = Worksheets("Lookup Sheet")
    Set rngLookup = wsLookup.Range("A1:B100")
    
    For Each c In Selection 'change as necessary
        c.Value = rngLookup.Find(what:=c.Value, lookat:=xlWhole).Offset(0, 1).Value
    Next c
End Sub