Age Band        Employee Count
------------------------------------------------------------
Under 18        8
18 - 24         11
25 - 31         11
32 - 38         15
39 - 45         14
46 - 52         15
53 - 54         4
55 Plus         71

Grouping people outside the bounds is easy. Simply check if the age is less than the lower or greater than or equal to the upper.

It's the bit in between which gets interesting :)

You can do this by:

1 Subtracting the lower bound from the employee's age ( 22 - 18 ) = 4
2 Dividing this by the gap value ( 4 / 7 ) = 0.56...
3 Taking the floor of the division ( floor ( 0.56... ) = 0
4 Multiplying this back up by ( 0 * 7 ) = 0
5 Adding the lower bound back on ( 0 + 18 ) = 18

To find the upper bound, repeat the process. But add one to the floor calculated in step 3. And, because the upper bound could be part way through a group, return the least of this calculation and the upper bound. Then subtract one from the result.

Stick all the above in a case expression. And group by the results of it to get the counts.

All that remains is to get the rows in the correct order. You can do this by:

Mapping Under ... to -1. And the other values to 0. The sorting by the group value, e.g.:

  order  by case 
    when grp like 'U%' then -1
    else 0
  end, grp

Which gives something like:

with bounds as (
  select 18 lwr, 55 upr, 7 gap from dual
), grps as (
  select a.*,
         case
           when employee_age < lwr then 'Under ' || lwr
           when employee_age >= upr then upr || ' Plus'
           else to_char ( ( floor ( ( employee_age - lwr ) / gap ) * gap ) + lwr ) || '-' ||
                to_char ( 
                  least ( 
                    upr, 
                    ( ( floor ( ( employee_age - lwr ) / gap ) + 1 ) * gap ) + lwr  
                  ) - 1
                )
         end grp
  from   a cross join bounds
  order  by employee_age
)
  select grp, count (*), 
         min ( employee_age ), max ( employee_age ) 
  from   grps
  group  by grp
  order  by case 
    when grp like 'U%' then -1
    else 0
  end, grp;

GRP        COUNT(*)   MIN(EMPLOYEE_AGE)   MAX(EMPLOYEE_AGE)   
Under 18            8                12.5                17.5 
18-24              11                18.5                24.5 
25-31              11                25.5                31.5 
32-38              15               32.25                38.5 
39-45              14                39.5                45.5 
46-52              15               46.25                52.5 
53-54               4                53.5                54.5 
55 Plus            71                55.5               100.5 

PS - please tell me you're not storing ages in the table! You're calculating this from stored birth dates, right? And you're not hiring 8 year-olds? ;)