From the following explanation it is clear that D is the only one whose age is an odd number, thus is the odd one out.
Final Arrangement:
Persons 
H 
E 
G 
C 
F 
B 
D 
A 
Age
(in years) 
7 
28 
10 
42 
18 
36 
13 
30 
Common Explanation:
Reference:
G is sitting second to left of the one who is 18 years old.
Two persons are sitting between G and B.
Person having age 13 is sitting immediate right of B.
Inference:
Following two cases are possible from the given hints.
Case1: When G is third to the left of B
Case2: When G is third to the right of B
Reference:
D is immediate neighbor of B.
G is as much younger than D as he is older than H.
None of G, H, and D are sitting immediate to one another.
Inference:
Given ages are  7, 10, 13, 18, 28, 30, 36, and 42
To satisfy the second hint now we'll check the possible combinations.
We have one such combination where age difference is equal throughout. 7, 10 and 13 (difference is 3)
Thus the ages of G, D and H are 10, 13 and 7 respectively.
Case1: When G is third to the left of B
Persons 
H 
G 
H 

B 
D 
Age 

10 

18 

13 
Case2: When G is third to the right of B
Persons 
B 
D 
H 
G 
H 

Age 

13 

10 

18 
Reference:
C is sitting third to right of the one who is 7 years old.
A is sitting second to right of the one who is 36 years old.
At least three persons are sitting between G and A.
Inference:
Case1: When G is third to the left of B
Persons 
H 

G 
C 

B 
D 
A 
Age 
7 

10 

18 
36 
13 

Case2: When G is third to the right of B
[Eliminated] because number of persons between A and G are not atleast 3.
Persons 
H 
B 
D 
C 
G 


A 
Age 
7 

13 

10 
36 


Reference:
F is sitting third to right of the one who is 10 years older than him.
A is not the oldest.
Inference:
The only possible place for F is to the immediate left of B. Then his age will be 18 years.
The vacant place is filled by the only left person i.e. E.
Thus as per the first hint age of E is 28 years.
As A is not the oldest, that means his age is not 42 years and the remaining ages are  42 and 30.
So, ages of A and C are 30 and 42 respectively.
Thus the final arrangement is as follows.
Case1: When G is third to the left of B
Persons 
H 
E 
G 
C 
F 
B 
D 
A 
Age 
7 
28 
10 
42 
18 
36 
13 
30 