declare @step int = 5;

declare @a table (age int, qty int)
insert into @a (age, qty) values 
	(18,10)
,	(18,15)
,	(20,5)
,	(22,10)
,	(31,14)
,	(35,6);

;with 
	b as 
( 
	select min(age) - min(age) % @step as g_age, max(age) + max(age) % @step as m_age from @a 
	union all
	select g_age + @step, m_age from b where g_age + @step <= m_age
)
,	c as 
(
	select g_age as min_age, isnull(lead(g_age) over (order by g_age), g_age + @step) as max_age from b
)
select concat(c.min_age, '-',c.max_age - 1) as age, sum(a.qty) as qty
from c
left join @a as a on a.age between c.min_age and c.max_age - 1
group by c.min_age, c.max_age;

declare @step int = 5;

declare @a table (age int, qty int)
insert into @a (age, qty) values 
	(18,10)
,	(18,15)
,	(20,5)
,	(22,10)
,	(31,14)
,	(35,6);

;with 
	b as 
( 
	select min(age) - min(age) % @step as beg_age, min(age) - min(age) % @step + @step as end_age
	,	max(age) + max(age) % @step as max_age from @a 
	union all
	select beg_age + @step,  beg_age + @step  + @step,  max_age from b where beg_age + @step <= max_age
)
select concat(b.beg_age, '-',b.end_age - 1) as age, sum(a.qty) as qty
from b
left join @a as a on a.age between b.beg_age and b.end_age - 1
group by b.beg_age, b.end_age;