Im 17 years old, am i legally allowed to drive on the highway by myself?

Yes, the issues surround who you're driving with. G2 for less than six months, you can only have one passenger under 20 years old when driving at night. G2 for more than six months, you can have up to three passengers under 20 years old.

The Nanny State, yours to discover

http://www.ontario.ca/laws/regulation/940340#BK0

6. (1) The holder of a Class G2 drivers licence may drive a Class G2 motor vehicle on a highway subject to the following conditions:

1. Revoked: O. Reg. 205/10, s. 3 (1).

2. The number of passengers in the motor vehicle must not exceed the number of operable seat belt assemblies installed in it.

3. Between the hours of midnight and 5 a.m., there must not be more than one passenger in the motor vehicle who is under the age of 20, other than a person who is a member of the novice drivers immediate family. O. Reg. 340/94, s. 6 (1); O. Reg. 196/05, s. 1 (1); O. Reg. 205/10, s. 3 (1).

(1.1) Despite subsection (1), the holder of a Class G2 drivers licence who has held a valid G2 drivers licence for at least six months may drive a Class G2 motor vehicle on a highway subject to the following conditions:

1. Revoked: O. Reg. 205/10, s. 3 (2).

2. The number of passengers in the motor vehicle must not exceed the number of operable seat belt assemblies installed in it.

3. Between the hours of midnight and 5 a.m., there must not be more than three passengers in the motor vehicle who are under the age of 20, other than a person who is a member of the novice drivers immediate family. O. Reg. 196/05, s. 1 (2); O. Reg. 205/10, s. 3 (2).

(3) For the purpose of subsection (1.1), the requirement that a novice driver has held a valid G2 licence for at least six months means that, at the time the novice driver is driving, he or she held a valid G2 licence for at least the immediately preceding six months. O. Reg. 196/05, s. 1 (4).

(4) Paragraph 3 of subsection (1) and paragraph 3 of subsection (1.1) do not apply to,

(a) a novice driver who is at least 20 years old; or

(b) any novice driver if a person who qualifies as an accompanying driver, as described in subsection 5 (2), and who meets the condition for an accompanying driver as set out in paragraph 2 of subsection 5 (1), occupies the seat beside the driver and no person other than the novice driver and the accompanying driver occupies a front seat in the motor vehicle. O. Reg. 196/05, s. 1 (4).

(5) The age distinctions in paragraph 3 of subsection (1), paragraph 3 of subsection (1.1) and clause (4) (a) apply despite the Human Rights Code. O. Reg. 196/05, s. 1 (4).

(6) In paragraph 3 of subsection (1) and paragraph 3 of subsection (1.1), immediate family includes the novice drivers guardian and immediate family who are related by blood, marriage, conjugal relationship outside marriage or adoption. O. Reg. 196/05, s. 1 (4).

iFly55 summed it up pretty well.

G1 drivers are not allowed on 400 series highways or expressways unless they are accompanied by a licensed driving instructor.

G2 drivers are allowed and encouraged to do so. Your G2 exit test will have a portion dedicated to it. Below is how you will be scored and deducted marks:

Entering

Traffic Check: No left/right mirrors, no blindspots

Signal: Late or never

Spacing: Not 2-3 seconds front/back/ out of acc. lane

Speed: too fast/slow/accelerate after acc. lane

Merge: Sharp/not to lane centre

Signal: on after 5 seconds

Driving Along

Traffic Check: not every 5 seconds

Speed: over limit/not traffic speed uneven/avoid obstacles

Spacing: under 2-3 seconds/sides/in blind spots/not inc. follow dist.

Exiting:

Traffic Check: No left/right mirrors, no blindspots

Signal: None/after exit lane

Exit lane: over line/after start of line/sharp turn to lane

Speed: slowed on expressway/spacing/rough slowing

Gap: under 2-3 seconds front

Signal: still on at ramp end