Theory:

     ‘Oh, just middle aged, but feeling young today.’ And to prove it he decided he’d climb over the wall instead of going round to the gate. He got up on the wall all right, but had to rest there, breathing heavily.

     ‘Middle-aged man on the flying trapeze,’ he muttered to himself.

     ‘I’ll help you,’ she said, and gave him her hand.

     He slithered down into a flower-bed, shattering the stem of a hollyhock.
Explanation:
 
He said maybe middle-aged, but he was young that day, and to prove that he decided to climb over the wall instead of going through the gate. Though he could climb up on the wall, he had to take some rest, as he was breathing heavily.
 
The man mumbled to himself that a middle-aged man like him is on a flying on a swing named trapeze.
 
She gave her hand and said she would help him climb the wall.
 
He wriggled into the flower bed, upsetting one of the stems of hollyhock.
 
Meanings of difficult words:
 
Number
Word
Meanings
1.
 Muttered  Whisper.
2.
 Trapeze A horizontal bar hanging by two ropes and free to swing.
3.
 Slithered  Glided.
4.
 Shattering  Upsetting.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. I Can’t Climb Trees Anymore - Ruskin Bond (Slightly adapted) (pp. 157-160). Published by the Tamil Nadu Textbook and Educational Services Corporation.