In the Harry Potter books, are we to assume the Mandrakes are sentient? They are portrayed as behaving much like human beings, such as going through adolescence, et cetera. However, if this is the case, chopping them up for potions seems more than a little harsh.
Yes, Mandrakes are sentient in their own way.
First, Mandrakes are described as "babies" in Chamber of Secrets -- Harry is surprised when Professor Sprout pulls the first Mandrake out of its pot and it looks humanesque. The Mandrake also apparently has a gender.
Harry let out a gasp of surprise that no one could hear.
Instead of roots, a small, muddy and extremely ugly baby popped out of the earth. The leaves were growing right out of his head. He had pale green, mottled skin, and was clearly bawling at the top of his lungs.
Chamber of Secrets - pages 72-73 - Bloomsbury - chapter six, Gilderoy Lockhart
The Mandrakes dislike being repotted and attempt to avoid being put into a new pot. This demonstrates they possess preferences.
Professor Sprout has to fit the Mandrakes with scarves in the winter which goes back to preference -- they are not immune to temperature preferences. They prefer being warm.
The sun had now begun to shine weakly on Hogwarts again. Inside the castle, the mood had grown more hopeful. There had been no more attacks since those on Justin and Nearly Headless Nick, and Madam Pomfrey was pleased to report that the Mandrakes were becoming moody and secretive, meaning that they were fast leaving childhood.
Chamber of Secrets - page 175 - Bloomsbury - chapter thirteen, The Very Secret Diary
"Moody" is a state of mind. A being cannot be moody without having a state of mind, i.e. sentience.
In March of Harry's second year, the Mandrakes throw a party in Greenhouse Three. This implies social interaction and a desire to have a shared experience with peers.
Many objects, creatures, and plants show sentience in Potterverse, where they would not in the Muggle world.
I think the question you are trying to ask is really if they are Sapient...
Many animals and plants are sentient but not sapient.
Sentience is the ability to feel, perceive, or to experience subjectivity.
Sapience is often defined as wisdom, or the ability of an organism or entity to act with appropriate judgement, a mental faculty which is a component of intelligence or alternatively may be considered an additional faculty, apart from intelligence, with its own properties.
As the previous person shows proof that they have forms of sentience, but I think you were trying to compare them to humans which would mean that you are wondering if they are human like aka Sapient, which they are not.
Just to clarify there is no proof in the books that the Mandrakes show forms of wisdom or judgement. This means that they are sentient but not sapient. If they were able to perform complex thought processes then yes they would be sapient but as I stated they do not show this in the book.
I also wanted to point out that Science Fiction in general tends to blur the lines between sentience / sapience. As the wiki states :
The words "sapience", "self-awareness", and "consciousness" are used in similar ways in science fiction