SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE tablename([AGE] int);
    
INSERT INTO tablename ([AGE]) VALUES
    (1),  (2),  (3),  (4),  (5),  (6),  (7),  (8), (9), (10), (11), (12), (13);
13 rows affected

SELECT AGE,
       CASE 
         WHEN AGE <= 3 THEN 3
         ELSE 3 + 2 + (AGE - 3 - 1) / 2 * 2
       END ADJ_AGE  
FROM tablename


AGE
ADJ_AGE


1
3

2
3

3
3

4
5

5
5

6
7

7
7

8
9

9
9

10
11

11
11

12
13

13
13


SELECT AGE,
       CASE 
         WHEN AGE <= 5 THEN 5
         ELSE 5 + 4 + (AGE - 5 - 1) / 4 * 4
       END ADJ_AGE  
FROM tablename


AGE
ADJ_AGE


1
5

2
5

3
5

4
5

5
5

6
9

7
9

8
9

9
9

10
13

11
13

12
13

13
13


SELECT AGE,
       CASE 
         WHEN AGE <= 4 THEN 4
         ELSE 4 + 3 + (AGE - 4 - 1) / 3 * 3
       END ADJ_AGE  
FROM tablename


AGE
ADJ_AGE


1
4

2
4

3
4

4
4

5
7

6
7

7
7

8
10

9
10

10
10

11
13

12
13

13
13


SELECT AGE,
       CASE 
         WHEN AGE <= 4 THEN 4
         ELSE 4 + 2 + (AGE - 4 - 1) / 2 * 2
       END ADJ_AGE  
FROM tablename


AGE
ADJ_AGE


1
4

2
4

3
4

4
4

5
6

6
6

7
8

8
8

9
10

10
10

11
12

12
12

13
14