Can a planeswalker be attacked the same turn it enters the battlefield?

Summoning sickness (CR 302.6) only restricts creatures from attacking or using tap/untap abilities the turn they enter the battlefield. Planeswalkers are not creatures (unless a card effect makes them so), so this restriction never applies to them.

When a player attacks, they declare attackers during the declare attackers step (CR 508.1). Attackers can be declared against the active player's opponents or any planeswalker those opponents control (CR 508.1d). There is no rule preventing a planeswalker that just entered the battlefield from being chosen as an attack target.

It is worth noting that the attacking player chooses which planeswalker (or opponent) each creature attacks — the defending player does not get to redirect attacks to a newly entered planeswalker; the attacker simply declares the planeswalker as the target from the start (CR 508.1d).

Concrete example: Your opponent plays Jace, the Mind Sculptor on their turn. On your very next turn, you can declare your creatures as attacking Jace directly, even though he entered the battlefield only moments ago. If enough combat damage is dealt to him, he will be destroyed.

The only protection a planeswalker has from immediate attack is if it has gained hexproof, shroud, or is protected by some other effect — not any inherent timing restriction on entering the battlefield.