You are welcome.

Actually I am saying that the E-minor waltz is more difficult (grade

than the Mendelssohn (grade 7).
And I don’t really know better than you. Difficulty is highly subjective. Maybe for you it is the other way round. Ultimately the difficulty of a piece is irrelevant, if you like it enough you will face whatever difficulty is there and master it.
so technically,how old are you when you're in grade 8?
because i'm 15 now and that is my current repertoire.
it would really help if you answered these questions too!
(thanks bernhard!!!)
Age has little to do with it. It is your ability (being able) to play that counts. Kissin was already playing Chopin concertos at age 12. Arrau was playing Liszt’s “Gnomenreigen” at age 7. And I have an 85 year old student who will be doing her grade 3 in 6 months time.
Also just playing advanced repertory is not enough. How much do you know about music in general? Can you analyse your pieces? Can you ascertain performance practices as related to style? For instance there is a huge difference in the techniques/resources you must use when performing a Bach fugue, from the techniques/resources you need to perform a Chopin’s waltz. Can you call upon these techniques/resources? Can you explain them - and “defend” cogently your way of interpreting these works?
In the Middle Ages there was a very clear distinction between three groups of people involved in the music business: the theoretician, the practitioner and the total musician. The theoretician was a music scholar. He understood in great depth the structure of music and how it all fitted together, but he could neither compose nor play. Because in the Middle Ages music was an integral part of education and was considered as an independent science for which neither performance nor composition was really necessary, he was highly respected (a bit like theoretical physicists and mathematicians today, who are not expected to build anything they may project). Then there was the practitioner. He could play music beautifully, but had no idea whatsoever about musical theory. This lack of knowledge was made up by an instinctive musical understanding. You can compare the theoretician and the practitioner with an specialist in linguistics – who knows everything about language and grammar – and a native speaker of that language – who can use it in a highly articulate manner, and yet may know nothing of grammar and syntax.
Finally you have the total musician, who not only understands and is fully conversant with the theory of music, as he can play and perform the music he understands so well. As a consequence, he can also compose. He is both theoretician and practitioner.
In the middle ages, a time where hierarchy suffused all parts of life, the total musician was at the top of the hierarchy, followed by the theoretician and finally by the practitioner.
So, do not concern yourself with grades and the like. Instead ask yourself who you are: practitioner, theoretician or total musician?
Of course, always aim at being a total musician.

Best wishes,
Bernhard.