Hi
for me it is based on internal information of a Forti Engineer which says " FG-100D" and above not for other smaler devices. Following link gives you another possibilitly to check:
http://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=11640&sliceId=1&docTypeID=DT_KCARTICLE_1_1&dialogID=59419836&stateId=0 0 59421286
Which means:
Link aggregation is also called Ethernet trunk, NIC teaming, port teaming, port trunking, and NIC bonding. Is link aggregation supported in FortiOS versions 4.00 MR2, 4.00 MR3 and 5.0.x?
--> Yes, but not necessarily on every FortiGate hardware platform.
How can I check if 802.3ad is supported on my FortiGate unit?
--> Create a new interface (System > Network > Interface) with a type of 802.3ad Aggregate. If this option does not appear, then link aggregation is not supported on your FortiGate unit
hope this helps
have fun
Andrea