Linksys WRT55AG giving IP addresses to any DHCP Client?

Is it true that even if i have WEP enabled for both my 2.4Ghz and
5Ghz systems, an exteral device can still get an IP address from my
DHCP server?

In other words, regardless if you have enabled all the security settings
on a wireless router, a wireless device can still get an IP address
from your wireless router?

I keep seeing a valid MAC address getting an IP address from my
WRT55AG router but the Expiration for the IP address given to it
seems not more than 60 seconds.

I couldn't get a reply from that device when i tried pinging it while
it still had a few seconds before it's IP expired on my DHCP Active
IP logs screen.

Any ideas?

Thank you