16 year old girls were expected to be maturer at that time. On their way to being well accomplished, and already showing signs of their maturer selves. Room for improvement is always there of course.

I can't imagine that 2 years before the start of the novel Elinor was in anyway as extreme as Marianne seems to be or as influenced by what she read without using her own intelligence.
At 16 Elinor must already have been showing signs of what she is now.