Mendelssohn almost seems like he was formed a complete composer from a very early age, and his style and complexity of his music didn't change dramatically his entire career. Of course he also didn't live past his 38th year, so no one knows if he would have changed his essentially conservative musical voice later in life.
The String Octet is from 1825 when Mendelssohn was sixteen years old, and another of his popular compositions, Overture To A Midsummer Night's Dream was written a year later. These were far from Mendelssohn's first works as he had written twelve symphonies for strings between the ages of 12-14. The octet is for a double string quartet; 4 violins, 2 violas and 2 cellos. Mendelssohn himself left directions for its performance: "This Octet must be played by all instruments in symphonic orchestral style. Pianos and fortes must be strictly observed and more strongly emphasized than usual." That the work was written with this orchestral style is evident from the very opening of the work, and that the work lends itself admirably to transcription for full string orchestra.
The octet is in 4 movements and opens with the first theme directly, played by violin with accompaniment. The first movement is far and away the longest in length, but Mendelssohn's inventiveness and mastery of sonata form keeps things interesting. The second movement is a study in gracefulness tinged with a tad of restlessness. It is the third movement, the scherzo where Mendelssohn shows hos much of a master he really was at only sixteen. Unlike almost all scherzos that are in 3/4 or triple time and ternary form, this one is in 2/4 time and sonata form. It is a precursor to the Overture To A Midsummer Night's Dream written the following year. It is taken at a rapid tempo at a subdued music, the original 'fairy' music of which Mendelssohn is known for. The finale begins as a fugue and also brings back echoes of the scherzo.
Mendelssohn was a musical conservative who tolerated the music of Wagner, Liszt , Berlioz and others of the new school, but he had no admiration of it. He helped found the Leipzig Conservatory which mirrored his own views on music and upheld the conservative tradition. But all of that makes not a hoot of difference as far as his music. It is all well-crafted, inspired and a delight to the ear.