I agree Doris - I am the same if out walking alone, always checking who is around me, particularly if I'm in a park or common area with trees, bushes, places where someone could hide.
Also I don't have an issue with her age guess - from the little I have seen of the defendant in those bin pics I would put him older than 16. And I have the benefit of a pic that I can look at again and again to assess his age, she only had a few glimpses, some of them side on.
Same with the height. If she was feeling a bit wary and apprehensive, she is not going to be accurately working out exactly how tall he is.
I think her evidence about him pulling up as though he had been running is quite significant. To me, that says he was running to get closer to her, but stopped when he realised she was aware of him.
None of the above means it was definitely the defendant of course but I think her evidence is fairly good, despite the Defence QC attempt to discredit it ( which he has done very well ).